from __future__ import unicode_literals

import copy
import sys
from functools import update_wrapper
from django.utils.six.moves import zip

import django.db.models.manager  # Imported to register signal handler.
from django.conf import settings
from django.core.exceptions import (ObjectDoesNotExist,
    MultipleObjectsReturned, FieldError, ValidationError, NON_FIELD_ERRORS)
from django.core import validators
from django.db.models.fields import AutoField, FieldDoesNotExist
from django.db.models.fields.related import (ManyToOneRel,
    OneToOneField, add_lazy_relation)
from django.db import (router, transaction, DatabaseError,
    DEFAULT_DB_ALIAS)
from django.db.models.query import Q
from django.db.models.query_utils import DeferredAttribute, deferred_class_factory
from django.db.models.deletion import Collector
from django.db.models.options import Options
from django.db.models import signals
from django.db.models.loading import register_models, get_model
from django.utils.translation import ugettext_lazy as _
from django.utils.functional import curry
from django.utils.encoding import force_str, force_text
from django.utils import six
from django.utils.text import get_text_list, capfirst


def subclass_exception(name, parents, module, attached_to=None):
    """
    Create exception subclass. Used by ModelBase below.

    If 'attached_to' is supplied, the exception will be created in a way that
    allows it to be pickled, assuming the returned exception class will be added
    as an attribute to the 'attached_to' class.
    """
    class_dict = {'__module__': module}
    if attached_to is not None:
        def __reduce__(self):
            # Exceptions are special - they've got state that isn't
            # in self.__dict__. We assume it is all in self.args.
            return (unpickle_inner_exception, (attached_to, name), self.args)

        def __setstate__(self, args):
            self.args = args

        class_dict['__reduce__'] = __reduce__
        class_dict['__setstate__'] = __setstate__

    return type(name, parents, class_dict)

ModelBase 是一种类型
class ModelBase(type):
    """
    Metaclass for all models.
    """
    def __new__(cls, name, bases, attrs):
        super_new = super(ModelBase, cls).__new__

        # six.with_metaclass() inserts an extra class called 'NewBase' in the
        # inheritance tree: Model -> NewBase -> object. But the initialization
        # should be executed only once for a given model class.

        # attrs will never be empty for classes declared in the standard way
        # (ie. with the `class` keyword). This is quite robust.
        if name == 'NewBase' and attrs == {}:
            return super_new(cls, name, bases, attrs)

        # Also ensure initialization is only performed for subclasses of Model
        # (excluding Model class itself).
        parents = [b for b in bases if isinstance(b, ModelBase) and
                not (b.__name__ == 'NewBase' and b.__mro__ == (b, object))]

        if not parents:
            return super_new(cls, name, bases, attrs)

        # Create the class.
        module = attrs.pop('__module__')
        new_class = super_new(cls, name, bases, {'__module__': module}) 新建类

        # 从参数 attrs 中获取 Meta 属性
        attr_meta = attrs.pop('Meta', None)

        abstract = getattr(attr_meta, 'abstract', False)

        if not attr_meta: 如果 attr_meta 为空
            """
            此处关于 Meta, 它是一个类, 可以在定义模块的时候, 添加需要的选项
            class OrderedPerson(Person):
                class Meta:
                    ordering = ["last_name"]
                    proxy = True

            更多关于 Meta 参见:
            https://docs.djangoproject.com/en/dev/topics/db/models/

            所以在这里 meta = new_class.Meta
            """
            meta = getattr(new_class, 'Meta', None)
        else:
            否则 meta 就是从参数 attrs 中获得的 Meta
            meta = attr_meta

        base_meta = getattr(new_class, '_meta', None)

        """
        有关 app_label 参见:
        Options.app_label
        If a model exists outside of the standard locations (models.py or a models package in an app), the model must define which app it is part of:
                `app_label = 'myapp'
        """

        if getattr(meta, 'app_label', None) is None:
            计算得出应用名
            # Figure out the app_label by looking one level up.
            # For 'django.contrib.sites.models', this would be 'sites'.
            model_module = sys.modules[new_class.__module__]
            kwargs = {"app_label": model_module.__name__.split('.')[-2]}
        else:
            kwargs = {}

        """new_class._meta 在这里添加, 值为 Options 对象"""
        new_class.add_to_class('_meta', Options(meta, **kwargs))

        if not abstract:
            new_class.add_to_class('DoesNotExist', subclass_exception(str('DoesNotExist'),
                    tuple(x.DoesNotExist
                          for x in parents if hasattr(x, '_meta') and not x._meta.abstract)
                    or (ObjectDoesNotExist,),
                    module, attached_to=new_class))

            new_class.add_to_class('MultipleObjectsReturned', subclass_exception(str('MultipleObjectsReturned'),
                    tuple(x.MultipleObjectsReturned
                          for x in parents if hasattr(x, '_meta') and not x._meta.abstract)
                    or (MultipleObjectsReturned,),
                    module, attached_to=new_class))

            if base_meta and not base_meta.abstract:
                # Non-abstract child classes inherit some attributes from their
                # non-abstract parent (unless an ABC comes before it in the
                # method resolution order).
                if not hasattr(meta, 'ordering'):
                    new_class._meta.ordering = base_meta.ordering

                if not hasattr(meta, 'get_latest_by'):
                    new_class._meta.get_latest_by = base_meta.get_latest_by

        # 更多关于 models proxy 请参见: https://docs.djangoproject.com/en/dev/topics/db/models/#proxy-models
        is_proxy = new_class._meta.proxy

        # If the model is a proxy, ensure that the base class
        # hasn't been swapped out.
        if is_proxy and base_meta and base_meta.swapped:
            raise TypeError("%s cannot proxy the swapped model '%s'." % (name, base_meta.swapped))

        # 每一个 model 都有一个 manager, 在 django.db.models.manager 中定义
        if getattr(new_class, '_default_manager', None):
            if not is_proxy:
                # Multi-table inheritance doesn't inherit default manager from
                # parents.
                new_class._default_manager = None
                new_class._base_manager = None
            else:
                # Proxy classes do inherit parent's default manager, if none is
                # set explicitly. 管理器继承自父类
                new_class._default_manager = new_class._default_manager._copy_to_model(new_class)
                new_class._base_manager = new_class._base_manager._copy_to_model(new_class)

        # Bail out early if we have already created this class.
        m = get_model(new_class._meta.app_label, name,
                      seed_cache=False, only_installed=False)
        if m is not None:
            return m

        将所有的属性都添加到 new_class 中
        # Add all attributes to the class.
        for obj_name, obj in attrs.items():
            new_class.add_to_class(obj_name, obj)

        # All the fields of any type declared on this model
        new_fields = new_class._meta.local_fields + \
                     new_class._meta.local_many_to_many + \
                     new_class._meta.virtual_fields
        field_names = set([f.name for f in new_fields])

        # Basic setup for proxy models.
        if is_proxy: 如果这个类的代理选项为真
            base = None

            for parent in [cls for cls in parents if hasattr(cls, '_meta')]:
                if parent._meta.abstract:
                    if parent._meta.fields:
                        raise TypeError("Abstract base class containing model fields not permitted for proxy model '%s'." % name)
                    else:
                        continue
                if base is not None:
                    raise TypeError("Proxy model '%s' has more than one non-abstract model base class." % name)
                else:
                    base = parent

            if base is None:
                    raise TypeError("Proxy model '%s' has no non-abstract model base class." % name)

            if (new_class._meta.local_fields or
                    new_class._meta.local_many_to_many):
                raise FieldError("Proxy model '%s' contains model fields." % name)

            new_class._meta.setup_proxy(base)
            new_class._meta.concrete_model = base._meta.concrete_model
        else:
            new_class._meta.concrete_model = new_class

        # Do the appropriate setup for any model parents.
        o2o_map = dict([(f.rel.to, f) for f in new_class._meta.local_fields
                if isinstance(f, OneToOneField)])

        将父类中的 fields 整合到 new_class 中
        for base in parents:
            original_base = base
            if not hasattr(base, '_meta'):
                # Things without _meta aren't functional models, so they're
                # uninteresting parents.
                continue

            parent_fields = base._meta.local_fields + base._meta.local_many_to_many
            # Check for clashes between locally declared fields and those
            # on the base classes (we cannot handle shadowed fields at the
            # moment).
            for field in parent_fields:
                if field.name in field_names:
                    raise FieldError('Local field %r in class %r clashes '
                                     'with field of similar name from '
                                     'base class %r' %
                                        (field.name, name, base.__name__))

            # 更多关于 meta.abstract 请参见: https://docs.djangoproject.com/en/dev/topics/db/models/#abstract-base-classes
            if not base._meta.abstract:
                # Concrete classes...
                base = base._meta.concrete_model

                if base in o2o_map:
                    field = o2o_map[base]

                elif not is_proxy:
                    # 创建一个指向父模块的属性
                    attr_name = '%s_ptr' % base._meta.module_name
                    field = OneToOneField(base, name=attr_name,
                            auto_created=True, parent_link=True)

                    # 放在子模块 new_class 中
                    new_class.add_to_class(attr_name, field)
                else:
                    field = None

                # 在子模块 new_class.parents 中添加此属性.
                new_class._meta.parents[base] = field
            else:
                # .. and abstract ones.
                for field in parent_fields:
                    new_class.add_to_class(field.name, copy.deepcopy(field))

                # Pass any non-abstract parent classes onto child.
                new_class._meta.parents.update(base._meta.parents)

            # Inherit managers from the abstract base classes.
            new_class.copy_managers(base._meta.abstract_managers)

            # Proxy models inherit the non-abstract managers from their base,
            # unless they have redefined any of them.
            if is_proxy:
                new_class.copy_managers(original_base._meta.concrete_managers)

            # Inherit virtual fields (like GenericForeignKey) from the parent
            # class
            for field in base._meta.virtual_fields:
                if base._meta.abstract and field.name in field_names:
                    raise FieldError('Local field %r in class %r clashes '\
                                     'with field of similar name from '\
                                     'abstract base class %r' % \
                                        (field.name, name, base.__name__))
                new_class.add_to_class(field.name, copy.deepcopy(field))

        if abstract:
            # Abstract base models can't be instantiated and don't appear in
            # the list of models for an app. We do the final setup for them a
            # little differently from normal models.
            attr_meta.abstract = False ???
            new_class.Meta = attr_meta
            return new_class

        new_class._prepare()
        register_models(new_class._meta.app_label, new_class)

        # Because of the way imports happen (recursively), we may or may not be
        # the first time this model tries to register with the framework. There
        # should only be one class for each model, so we always return the
        # registered version.
        return get_model(new_class._meta.app_label, name,
                         seed_cache=False, only_installed=False)

    非成员函数
    --------

    辅助 __new__
    def copy_managers(cls, base_managers):
        # This is in-place sorting of an Options attribute, but that's fine.
        base_managers.sort()
        for _, mgr_name, manager in base_managers:
            val = getattr(cls, mgr_name, None)
            if not val or val is manager:
                new_manager = manager._copy_to_model(cls)
                cls.add_to_class(mgr_name, new_manager)

    为 cls 设置值为 value 的 name 属性, 如果 value 有 contribute_to_class() 函数将会被调用.
    def add_to_class(cls, name, value):
        if hasattr(value, 'contribute_to_class'):
            value.contribute_to_class(cls, name)
        else:
            setattr(cls, name, value)

    def _prepare(cls):
        """
        Creates some methods once self._meta has been populated.
        """
        opts = cls._meta
        opts._prepare(cls)

        if opts.order_with_respect_to:
            cls.get_next_in_order = curry(cls._get_next_or_previous_in_order, is_next=True)
            cls.get_previous_in_order = curry(cls._get_next_or_previous_in_order, is_next=False)

            # defer creating accessors on the foreign class until we are
            # certain it has been created
            def make_foreign_order_accessors(field, model, cls):
                setattr(
                    field.rel.to,
                    'get_%s_order' % cls.__name__.lower(),
                    curry(method_get_order, cls)
                )
                setattr(
                    field.rel.to,
                    'set_%s_order' % cls.__name__.lower(),
                    curry(method_set_order, cls)
                )
            add_lazy_relation(
                cls,
                opts.order_with_respect_to,
                opts.order_with_respect_to.rel.to,
                make_foreign_order_accessors
            )

        # Give the class a docstring -- its definition. 创建一些说明文档
        if cls.__doc__ is None:
            cls.__doc__ = "%s(%s)" % (cls.__name__, ", ".join([f.attname for f in opts.fields]))

        if hasattr(cls, 'get_absolute_url'):
            cls.get_absolute_url = update_wrapper(curry(get_absolute_url, opts, cls.get_absolute_url),
                                                  cls.get_absolute_url)

        signals.class_prepared.send(sender=cls)


class ModelState(object):
    """
    A class for storing instance state
    """
    def __init__(self, db=None):
        self.db = db
        # If true, uniqueness validation checks will consider this a new, as-yet-unsaved object.
        如果为真, 唯一性检测对象将会被视为新的未被保存

        # Necessary for correct validation of new instances of objects with explicit (non-auto) PKs.
        # This impacts validation only; it has no effect on the actual save.
        self.adding = True

"""
!!! 在 django.db.models.manager 中有线索.

??? 在调试 django 的时候, 发现一个奇怪的现象: 在 eclipse 中看到如下

class Model(six.with_metaclass(ModelBase)):
    objects = django.db.models.manager.Manager()
    DoesNotExist = Exception
    MultipleObjectsReturned = Exception
    _deferred = False
    def __init__(self, *args, **kwargs):
        ....

但是明显下面的 Model 并没有定义前面三个成员变量, 不知道是什么时候加入的???

我们经常会用到下面的语句:

modelobject.objects.all() 来获取表中所有的数据, 其中 objects 就是上面 Model.objects 属性.
"""

class Model(six.with_metaclass(ModelBase)):
    _deferred = False

    def __init__(self, *args, **kwargs):
        signals.pre_init.send(sender=self.__class__, args=args, kwargs=kwargs)

        # Set up the storage for instance state
        self._state = ModelState()

        # There is a rather weird disparity 不一致 here; if kwargs, it's set, then args
        # overrides it. It should be one or the other; don't duplicate the work
        # The reason for the kwargs check is that standard iterator passes in by
        # args, and instantiation for iteration is 33% faster.

        # 上述说明是说, args 和 kwargs 可能会出现选项的重复, 应该尽量使用 kwargs, 因为迭代和实例化的速度都快

        args_len = len(args)
        if args_len > len(self._meta.fields):
            # Daft, but matches old exception sans the err msg.
            raise IndexError("Number of args exceeds number of fields")

        fields_iter = iter(self._meta.fields) fields 迭代器

        if not kwargs:
            # The ordering of the zip calls matter - zip throws StopIteration
            # when an iter throws it. So if the first iter throws it, the second
            # is *not* consumed. We rely on this, so don't change the order
            # without changing the logic.
            for val, field in zip(args, fields_iter):
                setattr(self, field.attname, val)
        else:
            # Slower, kwargs-ready version. 将 kwargs 中重复的的选项去掉
            for val, field in zip(args, fields_iter):
                setattr(self, field.attname, val)
                kwargs.pop(field.name, None)
                # Maintain compatibility with existing calls.
                if isinstance(field.rel, ManyToOneRel):
                    kwargs.pop(field.attname, None)

        # Now we're left with the unprocessed fields that *must* come from
        # keywords, or default.

        for field in fields_iter:
            is_related_object = False

            # This slightly odd construct is so that we can access any
            # data-descriptor object (DeferredAttribute) without triggering its
            # __get__ method.
            if (field.attname not in kwargs and
                    isinstance(self.__class__.__dict__.get(field.attname), DeferredAttribute)):
                # This field will be populated on request.
                continue

            if kwargs:
                if isinstance(field.rel, ManyToOneRel):
                    try:
                        # Assume object instance was passed in.
                        rel_obj = kwargs.pop(field.name)
                        is_related_object = True
                    except KeyError:
                        try:
                            # Object instance wasn't passed in -- must be an ID.
                            val = kwargs.pop(field.attname)
                        except KeyError:
                            val = field.get_default()
                    else:
                        # Object instance was passed in. Special case: You can
                        # pass in "None" for related objects if it's allowed.
                        if rel_obj is None and field.null:
                            val = None
                else:
                    try:
                        val = kwargs.pop(field.attname)
                    except KeyError:
                        # This is done with an exception rather than the
                        # default argument on pop because we don't want
                        # get_default() to be evaluated, and then not used.
                        # Refs #12057.
                        val = field.get_default()
            else:
                val = field.get_default()

            if is_related_object:
                # If we are passed a related instance, set it using the
                # field.name instead of field.attname (e.g. "user" instead of
                # "user_id") so that the object gets properly cached (and type
                # checked) by the RelatedObjectDescriptor.
                setattr(self, field.name, rel_obj)
            else:
                setattr(self, field.attname, val)

        if kwargs:
            for prop in list(kwargs):
                try:
                    if isinstance(getattr(self.__class__, prop), property):
                        setattr(self, prop, kwargs.pop(prop))
                except AttributeError:
                    pass
            if kwargs:
                raise TypeError("'%s' is an invalid keyword argument for this function" % list(kwargs)[0])
        super(Model, self).__init__()
        signals.post_init.send(sender=self.__class__, instance=self)

    def __repr__(self):
        try:
            u = six.text_type(self)
        except (UnicodeEncodeError, UnicodeDecodeError):
            u = '[Bad Unicode data]'
        return force_str('<%s: %s>' % (self.__class__.__name__, u))

    def __str__(self):
        if not six.PY3 and hasattr(self, '__unicode__'):
            if type(self).__unicode__ == Model.__str__:
                klass_name = type(self).__name__
                raise RuntimeError("%s.__unicode__ is aliased to __str__. Did"
                                   " you apply @python_2_unicode_compatible"
                                   " without defining __str__?" % klass_name)
            return force_text(self).encode('utf-8')
        return '%s object' % self.__class__.__name__

    def __eq__(self, other):
        return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val()

    def __ne__(self, other):
        return not self.__eq__(other)

    def __hash__(self):
        return hash(self._get_pk_val())

    def __reduce__(self):
        """
        Provides pickling support. Normally, this just dispatches to Python's
        standard handling. However, for models with deferred field loading, we
        need to do things manually, as they're dynamically created classes and
        only module-level classes can be pickled by the default path.
        """
        if not self._deferred:
            return super(Model, self).__reduce__()
        data = self.__dict__
        defers = []
        for field in self._meta.fields:
            if isinstance(self.__class__.__dict__.get(field.attname),
                    DeferredAttribute):
                defers.append(field.attname)
        model = self._meta.proxy_for_model
        return (model_unpickle, (model, defers), data)

    # 获取主键值, 主键值可能是未被设置的
    def _get_pk_val(self, meta=None):
        if not meta:
            meta = self._meta
        return getattr(self, meta.pk.attname)

    # 设置主键值, 等于是给 self 动态添加属性
    def _set_pk_val(self, value):
        return setattr(self, self._meta.pk.attname, value)

    descriptor
    pk = property(_get_pk_val, _set_pk_val)

    def serializable_value(self, field_name):
        """
        Returns the value of the field name for this instance. If the field is
        a foreign key, returns the id value, instead of the object. If there's
        no Field object with this name on the model, the model attribute's
        value is returned directly.

        Used to serialize a field's value (in the serializer, or form output,
        for example). Normally, you would just access the attribute directly
        and not use this method.
        """
        try:
            field = self._meta.get_field_by_name(field_name)[0]
        except FieldDoesNotExist:
            return getattr(self, field_name)
        return getattr(self, field.attname)

    def save(self, force_insert=False, force_update=False, using=None,
             update_fields=None):
        """
        Saves the current instance. Override this in a subclass if you want to
        control the saving process.

        The 'force_insert' and 'force_update' parameters can be used to insist
        that the "save" must be an SQL insert or update (or equivalent for
        non-SQL backends), respectively. Normally, they should not be set.

        一般来说, force_update 和 update_fi 集合elds 集合一般不会设置
        """
        using = using or router.db_for_write(self.__class__, instance=self)
        if force_insert and (force_update or update_fields):
            不能强制同时插入和更新
            raise ValueError("Cannot force both insert and updating in model saving.")

        if update_fields is not None:
            # If update_fields is empty, skip the save. We do also check for
            # no-op saves later on for inheritance cases. This bailout is
            # still needed for skipping signal sending.
            if len(update_fields) == 0:
                return

            update_fields = frozenset(update_fields)
            field_names = set()

            for field in self._meta.fields:

                # 如果 field 不是主键
                if not field.primary_key: !!!主键是不允许修改的
                    field_names.add(field.name)

                    # 当 Field.field_name 和 Field.attname 不同时, field 是外键
                    if field.name != field.attname:
                        field_names.add(field.attname)

            求出非 model 的表属性, 即不存在的非法属性
            non_model_fields = update_fields.difference(field_names)

            如果不为空, 异常
            if non_model_fields:
                raise ValueError("The following fields do not exist in this "
                                 "model or are m2m fields: %s"
                                 % ', '.join(non_model_fields))

        # If saving to the same database, and this model is deferred, then
        # automatically do a "update_fields" save on the loaded fields.
        model 的存储可能被延迟
        elif not force_insert and self._deferred and using == self._state.db:
            field_names = set()

            for field in self._meta.fields:
                # !!! 主键是不允许修改的
                if not field.primary_key and not hasattr(field, 'through'):
                    field_names.add(field.attname)

            算出需要推迟的表属性, 内部是如何实现的?
            deferred_fields = [
                f.attname for f in self._meta.fields
                if f.attname not in self.__dict__
                   and isinstance(self.__class__.__dict__[f.attname],
                                  DeferredAttribute)]

            # field_names - deferred_fields 得到需要立即存储的表属性
            loaded_fields = field_names.difference(deferred_fields)

            if loaded_fields:
                update_fields = frozenset(loaded_fields)

        self.save_base(using=using, force_insert=force_insert,
                       force_update=force_update, update_fields=update_fields)
    save.alters_data = True

    def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
                  force_update=False, using=None, update_fields=None):
        """
        子类不能重写

        Does the heavy-lifting 繁重的 involved in saving. Subclasses shouldn't need to
        override this method. It's separate from save() in order to hide the
        need for overrides of save() to pass around internal-only parameters
        ('raw', 'cls', and 'origin').
        """
        using = using or router.db_for_write(self.__class__, instance=self)
        assert not (force_insert and (force_update or update_fields))
        assert update_fields is None or len(update_fields) > 0

        if cls is None: 可以为空
            cls = self.__class__
            meta = cls._meta
            if not meta.proxy:
                origin = cls
        else:
            meta = cls._meta

        if origin and not meta.auto_created:
            signals.pre_save.send(sender=origin, instance=self, raw=raw, using=using,
                                  update_fields=update_fields)

        # If we are in a raw save, save the object exactly as presented.
        # That means that we don't try to be smart about saving attributes
        # that might have come from the parent class - we just save the
        # attributes we have been given to the class we have been given.
        # We also go through this process to defer the save of proxy objects
        # to their actual underlying model.
        if not raw or meta.proxy:

            if meta.proxy:
                org = cls
            else:
                org = None

            for parent, field in meta.parents.items():
                # At this point, parent's primary key field may be unknown
                # (for example, from administration form which doesn't fill
                # this field). If so, fill it.
                if field and getattr(self, parent._meta.pk.attname) is None and getattr(self, field.attname) is not None:
                    setattr(self, parent._meta.pk.attname, getattr(self, field.attname))

                对父类 model 执行 save_base() ???
                self.save_base(cls=parent, origin=org, using=using,
                               update_fields=update_fields)

                if field:
                    setattr(self, field.attname, self._get_pk_val(parent._meta))
                    # Since we didn't have an instance of the parent handy, we
                    # set attname directly, bypassing the descriptor.
                    # Invalidate the related object cache, in case it's been
                    # accidentally populated. A fresh instance will be
                    # re-built from the database if necessary.
                    cache_name = field.get_cache_name()
                    if hasattr(self, cache_name):
                        delattr(self, cache_name)

            if meta.proxy:
                return

        if not meta.proxy:
            non_pks = [f for f in meta.local_fields if not f.primary_key]

            if update_fields:
                non_pks = [f for f in non_pks if f.name in update_fields or f.attname in update_fields]

            # First, try an UPDATE. If that doesn't update anything, do an INSERT.
            # 直接获取主键的值, 如果没有设置, 将会返回 None
            pk_val = self._get_pk_val(meta)

            # pk_set 标记 pk_val 是否为空
            pk_set = pk_val is not None

            # 记录是否存在标记
            record_exists = True

            manager = cls._base_manager

            # 如果存在主键, 则执行更新操作
            if pk_set:
                # Determine if we should do an update (pk already exists, forced update,
                # no force_insert)
                if ((force_update or update_fields) or (not force_insert and
                        manager.using(using).filter(pk=pk_val).exists())):

                    if force_update or non_pks:
                        # 整理更新信息, Field.pre_save() 只返回需要更新的值
                        values = [(f, None, (raw and getattr(self, f.attname) or f.pre_save(self, False))) for f in non_pks]

                        if values:
                            # 执行更新
                            rows = manager.using(using).filter(pk=pk_val)._update(values)

                            # 强制跟新失败, 并没有影响任何的表项
                            if force_update and not rows:
                                raise DatabaseError("Forced update did not affect any rows.")

                            # update_fields 中的更新并没有影响任何的表项
                            if update_fields and not rows:
                                raise DatabaseError("Save with update_fields did not affect any rows.")
                else:
                    record_exists = False

            if not pk_set or not record_exists:
                if meta.order_with_respect_to:
                    # If this is a model with an order_with_respect_to
                    # autopopulate the _order field
                    field = meta.order_with_respect_to
                    order_value = manager.using(using).filter(**{field.name: getattr(self, field.attname)}).count()
                    self._order = order_value

                fields = meta.local_fields

                if not pk_set:
                    # 异常: 主键不存在又提供 force_update, 矛盾
                    if force_update or update_fields:
                        raise ValueError("Cannot force an update in save() with no primary key.")

                    # 筛选出表中自动填写的属性
                    fields = [f for f in fields if not isinstance(f, AutoField)]

                # 只好执行插入操作
                record_exists = False

                # 是否更新主键的值
                update_pk = bool(meta.has_auto_field and not pk_set)

                # 真正执行插入操作
                result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)

                if update_pk:
                    setattr(self, meta.pk.attname, result)

            # 数据库 commit
            transaction.commit_unless_managed(using=using)


        self._state.db = using
        # Once saved, this is no longer a to-be-added instance.
        self._state.adding = False

        # Signal that the save is complete
        if origin and not meta.auto_created:
            signals.post_save.send(sender=origin, instance=self, created=(not record_exists),
                                   update_fields=update_fields, raw=raw, using=using)

    save_base.alters_data = True

    # 借助 Collector 类实现
    def delete(self, using=None):
        using = using or router.db_for_write(self.__class__, instance=self)
        assert self._get_pk_val() is not None, "%s object can't be deleted because its %s attribute is set to None." % (self._meta.object_name, self._meta.pk.attname)

        借助了 Collector 类
        collector = Collector(using=using)
        collector.collect([self])
        collector.delete()

    delete.alters_data = True

    def _get_FIELD_display(self, field):
        value = getattr(self, field.attname)
        return force_text(dict(field.flatchoices).get(value, value), strings_only=True)

    def _get_next_or_previous_by_FIELD(self, field, is_next, **kwargs):
        if not self.pk:
            raise ValueError("get_next/get_previous cannot be used on unsaved objects.")

        op = is_next and 'gt' or 'lt'
        order = not is_next and '-' or '' 排序
        param = force_text(getattr(self, field.attname))

        q = Q(**{'%s__%s' % (field.name, op): param})
        q = q | Q(**{field.name: param, 'pk__%s' % op: self.pk})
        qs = self.__class__._default_manager.using(self._state.db).filter(**kwargs).filter(q).order_by('%s%s' % (order, field.name), '%spk' % order)

        try:
            return qs[0]

        except IndexError:
            raise self.DoesNotExist("%s matching query does not exist." % self.__class__._meta.object_name)

    def _get_next_or_previous_in_order(self, is_next):
        cachename = "__%s_order_cache" % is_next

        if not hasattr(self, cachename):
            op = is_next and 'gt' or 'lt'
            order = not is_next and '-_order' or '_order'
            order_field = self._meta.order_with_respect_to

            obj = self._default_manager.filter(**{
                order_field.name: getattr(self, order_field.attname)
            }).filter(**{
                '_order__%s' % op: self._default_manager.values('_order').filter(**{
                    self._meta.pk.name: self.pk
                })
            }).order_by(order)[:1].get()
            setattr(self, cachename, obj)
        return getattr(self, cachename)

    def prepare_database_save(self, unused):
        return self.pk

    def clean(self):
        """
        Hook for doing any extra model-wide validation after clean() has been
        called on every field by self.clean_fields. Any ValidationError raised
        by this method will not be associated with a particular field; it will
        have a special-case association with the field defined by NON_FIELD_ERRORS.
        """
        pass

    def validate_unique(self, exclude=None):
        """
        Checks unique constraints on the model and raises ``ValidationError``
        if any failed.
        """
        unique_checks, date_checks = self._get_unique_checks(exclude=exclude)

        errors = self._perform_unique_checks(unique_checks)
        date_errors = self._perform_date_checks(date_checks)

        for k, v in date_errors.items():
            errors.setdefault(k, []).extend(v)

        if errors:
            raise ValidationError(errors)

    def _get_unique_checks(self, exclude=None):
        """
        Gather a list of checks to perform. Since validate_unique could be
        called from a ModelForm, some fields may have been excluded; we can't
        perform a unique check on a model that is missing fields involved
        in that check.
        Fields that did not validate should also be excluded, but they need
        to be passed in via the exclude argument.
        """
        if exclude is None: exclude 可能为空, 但必须把它转为 list
            exclude = []

        # 唯一性检测
        unique_checks = []

        unique_togethers = [(self.__class__, self._meta.unique_together)]
        for parent_class in self._meta.parents.keys():
            if parent_class._meta.unique_together:
                unique_togethers.append((parent_class, parent_class._meta.unique_together))

        for model_class, unique_together in unique_togethers:
            for check in unique_together:
                for name in check:
                    # If this is an excluded field, don't add this check.
                    if name in exclude:
                        break
                else:
                    unique_checks.append((model_class, tuple(check)))

        # 日期检测
        # These are checks for the unique_for_<date/year/month>.
        date_checks = []

        # Gather a list of checks for fields declared as unique and add them to
        # the list of checks.

        fields_with_class = [(self.__class__, self._meta.local_fields)]
        for parent_class in self._meta.parents.keys():
            fields_with_class.append((parent_class, parent_class._meta.local_fields))

        for model_class, fields in fields_with_class:
            for f in fields:
                name = f.name
                if name in exclude:
                    continue
                if f.unique:
                    unique_checks.append((model_class, (name,)))
                if f.unique_for_date and f.unique_for_date not in exclude:
                    date_checks.append((model_class, 'date', name, f.unique_for_date))
                if f.unique_for_year and f.unique_for_year not in exclude:
                    date_checks.append((model_class, 'year', name, f.unique_for_year))
                if f.unique_for_month and f.unique_for_month not in exclude:
                    date_checks.append((model_class, 'month', name, f.unique_for_month))
        return unique_checks, date_checks

    def _perform_unique_checks(self, unique_checks):
        errors = {}

        for model_class, unique_check in unique_checks:
            # Try to look up an existing object with the same values as this
            # object's values for all the unique field.

            lookup_kwargs = {}
            for field_name in unique_check:
                f = self._meta.get_field(field_name)
                lookup_value = getattr(self, f.attname)

                if lookup_value is None:
                    # no value, skip the lookup
                    continue

                if f.primary_key and not self._state.adding:
                    # no need to check for unique primary key when editing
                    continue

                加入到表中
                lookup_kwargs[str(field_name)] = lookup_value

            # some fields were skipped, no reason to do the check
            if len(unique_check) != len(lookup_kwargs):
                continue

            qs = model_class._default_manager.filter(**lookup_kwargs)

            # Exclude the current object from the query if we are editing an
            # instance (as opposed to creating a new one)
            # Note that we need to use the pk as defined by model_class, not
            # self.pk. These can be different fields because model inheritance
            # allows single model to have effectively multiple primary keys.
            # Refs #17615.
            model_class_pk = self._get_pk_val(model_class._meta)
            if not self._state.adding and model_class_pk is not None:
                qs = qs.exclude(pk=model_class_pk)

            if qs.exists():
                if len(unique_check) == 1:
                    key = unique_check[0]
                else:
                    key = NON_FIELD_ERRORS
                errors.setdefault(key, []).append(self.unique_error_message(model_class, unique_check))

        return errors

    def _perform_date_checks(self, date_checks):
        errors = {}
        for model_class, lookup_type, field, unique_for in date_checks:
            lookup_kwargs = {}
            # there's a ticket to add a date lookup, we can remove this special
            # case if that makes it's way in
            date = getattr(self, unique_for)
            if date is None:
                continue
            if lookup_type == 'date':
                lookup_kwargs['%s__day' % unique_for] = date.day
                lookup_kwargs['%s__month' % unique_for] = date.month
                lookup_kwargs['%s__year' % unique_for] = date.year
            else:
                lookup_kwargs['%s__%s' % (unique_for, lookup_type)] = getattr(date, lookup_type)
            lookup_kwargs[field] = getattr(self, field)

            qs = model_class._default_manager.filter(**lookup_kwargs)
            # Exclude the current object from the query if we are editing an
            # instance (as opposed to creating a new one)
            if not self._state.adding and self.pk is not None:
                qs = qs.exclude(pk=self.pk)

            if qs.exists():
                errors.setdefault(field, []).append(
                    self.date_error_message(lookup_type, field, unique_for)
                )
        return errors

    def date_error_message(self, lookup_type, field, unique_for):
        opts = self._meta
        return _("%(field_name)s must be unique for %(date_field)s %(lookup)s.") % {
            'field_name': six.text_type(capfirst(opts.get_field(field).verbose_name)),
            'date_field': six.text_type(capfirst(opts.get_field(unique_for).verbose_name)),
            'lookup': lookup_type,
        }

    def unique_error_message(self, model_class, unique_check):
        opts = model_class._meta
        model_name = capfirst(opts.verbose_name)

        # A unique field
        if len(unique_check) == 1:
            field_name = unique_check[0]
            field = opts.get_field(field_name)
            field_label = capfirst(field.verbose_name)
            # Insert the error into the error dict, very sneaky
            return field.error_messages['unique'] % {
                'model_name': six.text_type(model_name),
                'field_label': six.text_type(field_label)
            }
        # unique_together
        else:
            field_labels = [capfirst(opts.get_field(f).verbose_name) for f in unique_check]
            field_labels = get_text_list(field_labels, _('and'))
            return _("%(model_name)s with this %(field_label)s already exists.") % {
                'model_name': six.text_type(model_name),
                'field_label': six.text_type(field_labels)
            }

    执行所有的检测
    def full_clean(self, exclude=None):
        """
        Calls clean_fields, clean, and validate_unique, on the model,
        and raises a ``ValidationError`` for any errors that occured.
        """
        errors = {}
        if exclude is None:
            exclude = []

        try:
            self.clean_fields(exclude=exclude)
        except ValidationError as e:
            errors = e.update_error_dict(errors)

        # Form.clean() is run even if other validation fails, so do the
        # same with Model.clean() for consistency.
        try:
            self.clean()
        except ValidationError as e:
            errors = e.update_error_dict(errors)

        # Run unique checks, but only for fields that passed validation.
        for name in errors.keys():
            if name != NON_FIELD_ERRORS and name not in exclude:
                exclude.append(name)

        try:
            self.validate_unique(exclude=exclude)
        except ValidationError as e:
            errors = e.update_error_dict(errors)

        if errors:
            raise ValidationError(errors)

    def clean_fields(self, exclude=None):
        """
        Cleans all fields and raises a ValidationError containing message_dict
        of all validation errors if any occur.
        """
        if exclude is None:
            exclude = []

        errors = {}
        for f in self._meta.fields:
            if f.name in exclude:
                continue

            # Skip validation for empty fields with blank=True. The developer
            # is responsible for making sure they have a valid value.
            raw_value = getattr(self, f.attname)
            if f.blank and raw_value in validators.EMPTY_VALUES: 空的
                continue
            try:
                setattr(self, f.attname, f.clean(raw_value, self))
            except ValidationError as e:
                errors[f.name] = e.messages

        if errors:
            raise ValidationError(errors)


############################################
# HELPER FUNCTIONS (CURRIED MODEL METHODS) #
############################################

# ORDERING METHODS #########################

def method_set_order(ordered_obj, self, id_list, using=None):
    if using is None:
        using = DEFAULT_DB_ALIAS
    rel_val = getattr(self, ordered_obj._meta.order_with_respect_to.rel.field_name)
    order_name = ordered_obj._meta.order_with_respect_to.name
    # FIXME: It would be nice if there was an "update many" version of update
    # for situations like this.
    for i, j in enumerate(id_list):
        ordered_obj.objects.filter(**{'pk': j, order_name: rel_val}).update(_order=i)
    transaction.commit_unless_managed(using=using)


def method_get_order(ordered_obj, self):
    rel_val = getattr(self, ordered_obj._meta.order_with_respect_to.rel.field_name)
    order_name = ordered_obj._meta.order_with_respect_to.name
    pk_name = ordered_obj._meta.pk.name
    return [r[pk_name] for r in
            ordered_obj.objects.filter(**{order_name: rel_val}).values(pk_name)]


##############################################
# HELPER FUNCTIONS (CURRIED MODEL FUNCTIONS) #
##############################################

def get_absolute_url(opts, func, self, *args, **kwargs):
    return settings.ABSOLUTE_URL_OVERRIDES.get('%s.%s' % (opts.app_label, opts.module_name), func)(self, *args, **kwargs)


########
# MISC #
########

class Empty(object):
    pass


def model_unpickle(model, attrs):
    """
    Used to unpickle Model subclasses with deferred fields.
    """
    cls = deferred_class_factory(model, attrs)
    return cls.__new__(cls)
model_unpickle.__safe_for_unpickle__ = True


def unpickle_inner_exception(klass, exception_name):
    # Get the exception class from the class it is attached to:
    exception = getattr(klass, exception_name)
    return exception.__new__(exception)
