text: # ext/hybrid.py
  # Copyright (C) 2005-2020 the SQLAlchemy authors and contributors
  # <see AUTHORS file>
  #
  # This module is part of SQLAlchemy and is released under
  # the MIT License: http://www.opensource.org/licenses/mit-license.php
  
  r"""Define attributes on ORM-mapped classes that have "hybrid" behavior.
  
  "hybrid" means the attribute has distinct behaviors defined at the
  class level and at the instance level.
  
  The :mod:`~sqlalchemy.ext.hybrid` extension provides a special form of
  method decorator, is around 50 lines of code and has almost no
  dependencies on the rest of SQLAlchemy.  It can, in theory, work with
  any descriptor-based expression system.
  
  Consider a mapping ``Interval``, representing integer ``start`` and ``end``
  values. We can define higher level functions on mapped classes that produce SQL
  expressions at the class level, and Python expression evaluation at the
  instance level.  Below, each function decorated with :class:`.hybrid_method` or
  :class:`.hybrid_property` may receive ``self`` as an instance of the class, or
  as the class itself::
  
      from sqlalchemy import Column, Integer
      from sqlalchemy.ext.declarative import declarative_base
      from sqlalchemy.orm import Session, aliased
      from sqlalchemy.ext.hybrid import hybrid_property, hybrid_method
  
      Base = declarative_base()
  
      class Interval(Base):
          __tablename__ = 'interval'
  
          id = Column(Integer, primary_key=True)
          start = Column(Integer, nullable=False)
          end = Column(Integer, nullable=False)
  
          def __init__(self, start, end):
              self.start = start
              self.end = end
  
          @hybrid_property
          def length(self):
              return self.end - self.start
  
          @hybrid_method
          def contains(self, point):
              return (self.start <= point) & (point <= self.end)
  
          @hybrid_method
          def intersects(self, other):
              return self.contains(other.start) | self.contains(other.end)
  
  Above, the ``length`` property returns the difference between the
  ``end`` and ``start`` attributes.  With an instance of ``Interval``,
  this subtraction occurs in Python, using normal Python descriptor
  mechanics::
  
      >>> i1 = Interval(5, 10)
      >>> i1.length
      5
  
  When dealing with the ``Interval`` class itself, the :class:`.hybrid_property`
  descriptor evaluates the function body given the ``Interval`` class as
  the argument, which when evaluated with SQLAlchemy expression mechanics
  (here using the :attr:`.QueryableAttribute.expression` accessor)
  returns a new SQL expression::
  
      >>> print(Interval.length.expression)
      interval."end" - interval.start
  
      >>> print(Session().query(Interval).filter(Interval.length > 10))
      SELECT interval.id AS interval_id, interval.start AS interval_start,
      interval."end" AS interval_end
      FROM interval
      WHERE interval."end" - interval.start > :param_1
  
  ORM methods such as :meth:`_query.Query.filter_by`
  generally use ``getattr()`` to
  locate attributes, so can also be used with hybrid attributes::
  
      >>> print(Session().query(Interval).filter_by(length=5))
      SELECT interval.id AS interval_id, interval.start AS interval_start,
      interval."end" AS interval_end
      FROM interval
      WHERE interval."end" - interval.start = :param_1
  
  The ``Interval`` class example also illustrates two methods,
  ``contains()`` and ``intersects()``, decorated with
  :class:`.hybrid_method`. This decorator applies the same idea to
  methods that :class:`.hybrid_property` applies to attributes.   The
  methods return boolean values, and take advantage of the Python ``|``
  and ``&`` bitwise operators to produce equivalent instance-level and
  SQL expression-level boolean behavior::
  
      >>> i1.contains(6)
      True
      >>> i1.contains(15)
      False
      >>> i1.intersects(Interval(7, 18))
      True
      >>> i1.intersects(Interval(25, 29))
      False
  
      >>> print(Session().query(Interval).filter(Interval.contains(15)))
      SELECT interval.id AS interval_id, interval.start AS interval_start,
      interval."end" AS interval_end
      FROM interval
      WHERE interval.start <= :start_1 AND interval."end" > :end_1
  
      >>> ia = aliased(Interval)
      >>> print(Session().query(Interval, ia).filter(Interval.intersects(ia)))
      SELECT interval.id AS interval_id, interval.start AS interval_start,
      interval."end" AS interval_end, interval_1.id AS interval_1_id,
      interval_1.start AS interval_1_start, interval_1."end" AS interval_1_end
      FROM interval, interval AS interval_1
      WHERE interval.start <= interval_1.start
          AND interval."end" > interval_1.start
          OR interval.start <= interval_1."end"
          AND interval."end" > interval_1."end"
  
  .. _hybrid_distinct_expression:
  
  Defining Expression Behavior Distinct from Attribute Behavior
  --------------------------------------------------------------
  
  Our usage of the ``&`` and ``|`` bitwise operators above was
  fortunate, considering our functions operated on two boolean values to
  return a new one.   In many cases, the construction of an in-Python
  function and a SQLAlchemy SQL expression have enough differences that
  two separate Python expressions should be defined.  The
  :mod:`~sqlalchemy.ext.hybrid` decorators define the
  :meth:`.hybrid_property.expression` modifier for this purpose.   As an
  example we'll define the radius of the interval, which requires the
  usage of the absolute value function::
  
      from sqlalchemy import func
  
      class Interval(object):
          # ...
  
          @hybrid_property
          def radius(self):
              return abs(self.length) / 2
  
          @radius.expression
          def radius(cls):
              return func.abs(cls.length) / 2
  
  Above the Python function ``abs()`` is used for instance-level
  operations, the SQL function ``ABS()`` is used via the :data:`.func`
  object for class-level expressions::
  
      >>> i1.radius
      2
  
      >>> print(Session().query(Interval).filter(Interval.radius > 5))
      SELECT interval.id AS interval_id, interval.start AS interval_start,
          interval."end" AS interval_end
      FROM interval
      WHERE abs(interval."end" - interval.start) / :abs_1 > :param_1
  
  .. note:: When defining an expression for a hybrid property or method, the
     expression method **must** retain the name of the original hybrid, else
     the new hybrid with the additional state will be attached to the class
     with the non-matching name. To use the example above::
  
      class Interval(object):
          # ...
  
          @hybrid_property
          def radius(self):
              return abs(self.length) / 2
  
          # WRONG - the non-matching name will cause this function to be
          # ignored
          @radius.expression
          def radius_expression(cls):
              return func.abs(cls.length) / 2
  
     This is also true for other mutator methods, such as
     :meth:`.hybrid_property.update_expression`. This is the same behavior
     as that of the ``@property`` construct that is part of standard Python.
  
  Defining Setters
  ----------------
  
  Hybrid properties can also define setter methods.  If we wanted
  ``length`` above, when set, to modify the endpoint value::
  
      class Interval(object):
          # ...
  
          @hybrid_property
          def length(self):
              return self.end - self.start
  
          @length.setter
          def length(self, value):
              self.end = self.start + value
  
  The ``length(self, value)`` method is now called upon set::
  
      >>> i1 = Interval(5, 10)
      >>> i1.length
      5
      >>> i1.length = 12
      >>> i1.end
      17
  
  .. _hybrid_bulk_update:
  
  Allowing Bulk ORM Update
  ------------------------
  
  A hybrid can define a custom "UPDATE" handler for when using the
  :meth:`_query.Query.update` method, allowing the hybrid to be used in the
  SET clause of the update.
  
  Normally, when using a hybrid with :meth:`_query.Query.update`, the SQL
  expression is used as the column that's the target of the SET.  If our
  ``Interval`` class had a hybrid ``start_point`` that linked to
  ``Interval.start``, this could be substituted directly::
  
      session.query(Interval).update({Interval.start_point: 10})
  
  However, when using a composite hybrid like ``Interval.length``, this
  hybrid represents more than one column.   We can set up a handler that will
  accommodate a value passed to :meth:`_query.Query.update` which can affect
  this, using the :meth:`.hybrid_property.update_expression` decorator.
  A handler that works similarly to our setter would be::
  
      class Interval(object):
          # ...
  
          @hybrid_property
          def length(self):
              return self.end - self.start
  
          @length.setter
          def length(self, value):
              self.end = self.start + value
  
          @length.update_expression
          def length(cls, value):
              return [
                  (cls.end, cls.start + value)
              ]
  
  Above, if we use ``Interval.length`` in an UPDATE expression as::
  
      session.query(Interval).update(
          {Interval.length: 25}, synchronize_session='fetch')
  
  We'll get an UPDATE statement along the lines of::
  
      UPDATE interval SET end=start + :value
  
  In some cases, the default "evaluate" strategy can't perform the SET
  expression in Python; while the addition operator we're using above
  is supported, for more complex SET expressions it will usually be necessary
  to use either the "fetch" or False synchronization strategy as illustrated
  above.
  
  .. versionadded:: 1.2 added support for bulk updates to hybrid properties.
  
  Working with Relationships
  --------------------------
  
  There's no essential difference when creating hybrids that work with
  related objects as opposed to column-based data. The need for distinct
  expressions tends to be greater.  The two variants we'll illustrate
  are the "join-dependent" hybrid, and the "correlated subquery" hybrid.
  
  Join-Dependent Relationship Hybrid
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  Consider the following declarative
  mapping which relates a ``User`` to a ``SavingsAccount``::
  
      from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
      from sqlalchemy.orm import relationship
      from sqlalchemy.ext.declarative import declarative_base
      from sqlalchemy.ext.hybrid import hybrid_property
  
      Base = declarative_base()
  
      class SavingsAccount(Base):
          __tablename__ = 'account'
          id = Column(Integer, primary_key=True)
          user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
          balance = Column(Numeric(15, 5))
  
      class User(Base):
          __tablename__ = 'user'
          id = Column(Integer, primary_key=True)
          name = Column(String(100), nullable=False)
  
          accounts = relationship("SavingsAccount", backref="owner")
  
          @hybrid_property
          def balance(self):
              if self.accounts:
                  return self.accounts[0].balance
              else:
                  return None
  
          @balance.setter
          def balance(self, value):
              if not self.accounts:
                  account = Account(owner=self)
              else:
                  account = self.accounts[0]
              account.balance = value
  
          @balance.expression
          def balance(cls):
              return SavingsAccount.balance
  
  The above hybrid property ``balance`` works with the first
  ``SavingsAccount`` entry in the list of accounts for this user.   The
  in-Python getter/setter methods can treat ``accounts`` as a Python
  list available on ``self``.
  
  However, at the expression level, it's expected that the ``User`` class will
  be used in an appropriate context such that an appropriate join to
  ``SavingsAccount`` will be present::
  
      >>> print(Session().query(User, User.balance).
      ...       join(User.accounts).filter(User.balance > 5000))
      SELECT "user".id AS user_id, "user".name AS user_name,
      account.balance AS account_balance
      FROM "user" JOIN account ON "user".id = account.user_id
      WHERE account.balance > :balance_1
  
  Note however, that while the instance level accessors need to worry
  about whether ``self.accounts`` is even present, this issue expresses
  itself differently at the SQL expression level, where we basically
  would use an outer join::
  
      >>> from sqlalchemy import or_
      >>> print (Session().query(User, User.balance).outerjoin(User.accounts).
      ...         filter(or_(User.balance < 5000, User.balance == None)))
      SELECT "user".id AS user_id, "user".name AS user_name,
      account.balance AS account_balance
      FROM "user" LEFT OUTER JOIN account ON "user".id = account.user_id
      WHERE account.balance <  :balance_1 OR account.balance IS NULL
  
  Correlated Subquery Relationship Hybrid
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  We can, of course, forego being dependent on the enclosing query's usage
  of joins in favor of the correlated subquery, which can portably be packed
  into a single column expression. A correlated subquery is more portable, but
  often performs more poorly at the SQL level. Using the same technique
  illustrated at :ref:`mapper_column_property_sql_expressions`,
  we can adjust our ``SavingsAccount`` example to aggregate the balances for
  *all* accounts, and use a correlated subquery for the column expression::
  
      from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
      from sqlalchemy.orm import relationship
      from sqlalchemy.ext.declarative import declarative_base
      from sqlalchemy.ext.hybrid import hybrid_property
      from sqlalchemy import select, func
  
      Base = declarative_base()
  
      class SavingsAccount(Base):
          __tablename__ = 'account'
          id = Column(Integer, primary_key=True)
          user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
          balance = Column(Numeric(15, 5))
  
      class User(Base):
          __tablename__ = 'user'
          id = Column(Integer, primary_key=True)
          name = Column(String(100), nullable=False)
  
          accounts = relationship("SavingsAccount", backref="owner")
  
          @hybrid_property
          def balance(self):
              return sum(acc.balance for acc in self.accounts)
  
          @balance.expression
          def balance(cls):
              return select([func.sum(SavingsAccount.balance)]).\
                      where(SavingsAccount.user_id==cls.id).\
                      label('total_balance')
  
  The above recipe will give us the ``balance`` column which renders
  a correlated SELECT::
  
      >>> print(s.query(User).filter(User.balance > 400))
      SELECT "user".id AS user_id, "user".name AS user_name
      FROM "user"
      WHERE (SELECT sum(account.balance) AS sum_1
      FROM account
      WHERE account.user_id = "user".id) > :param_1
  
  .. _hybrid_custom_comparators:
  
  Building Custom Comparators
  ---------------------------
  
  The hybrid property also includes a helper that allows construction of
  custom comparators. A comparator object allows one to customize the
  behavior of each SQLAlchemy expression operator individually.  They
  are useful when creating custom types that have some highly
  idiosyncratic behavior on the SQL side.
  
  .. note::  The :meth:`.hybrid_property.comparator` decorator introduced
     in this section **replaces** the use of the
     :meth:`.hybrid_property.expression` decorator.
     They cannot be used together.
  
  The example class below allows case-insensitive comparisons on the attribute
  named ``word_insensitive``::
  
      from sqlalchemy.ext.hybrid import Comparator, hybrid_property
      from sqlalchemy import func, Column, Integer, String
      from sqlalchemy.orm import Session
      from sqlalchemy.ext.declarative import declarative_base
  
      Base = declarative_base()
  
      class CaseInsensitiveComparator(Comparator):
          def __eq__(self, other):
              return func.lower(self.__clause_element__()) == func.lower(other)
  
      class SearchWord(Base):
          __tablename__ = 'searchword'
          id = Column(Integer, primary_key=True)
          word = Column(String(255), nullable=False)
  
          @hybrid_property
          def word_insensitive(self):
              return self.word.lower()
  
          @word_insensitive.comparator
          def word_insensitive(cls):
              return CaseInsensitiveComparator(cls.word)
  
  Above, SQL expressions against ``word_insensitive`` will apply the ``LOWER()``
  SQL function to both sides::
  
      >>> print(Session().query(SearchWord).filter_by(word_insensitive="Trucks"))
      SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
      FROM searchword
      WHERE lower(searchword.word) = lower(:lower_1)
  
  The ``CaseInsensitiveComparator`` above implements part of the
  :class:`.ColumnOperators` interface.   A "coercion" operation like
  lowercasing can be applied to all comparison operations (i.e. ``eq``,
  ``lt``, ``gt``, etc.) using :meth:`.Operators.operate`::
  
      class CaseInsensitiveComparator(Comparator):
          def operate(self, op, other):
              return op(func.lower(self.__clause_element__()), func.lower(other))
  
  .. _hybrid_reuse_subclass:
  
  Reusing Hybrid Properties across Subclasses
  -------------------------------------------
  
  A hybrid can be referred to from a superclass, to allow modifying
  methods like :meth:`.hybrid_property.getter`, :meth:`.hybrid_property.setter`
  to be used to redefine those methods on a subclass.  This is similar to
  how the standard Python ``@property`` object works::
  
      class FirstNameOnly(Base):
          # ...
  
          first_name = Column(String)
  
          @hybrid_property
          def name(self):
              return self.first_name
  
          @name.setter
          def name(self, value):
              self.first_name = value
  
      class FirstNameLastName(FirstNameOnly):
          # ...
  
          last_name = Column(String)
  
          @FirstNameOnly.name.getter
          def name(self):
              return self.first_name + ' ' + self.last_name
  
          @name.setter
          def name(self, value):
              self.first_name, self.last_name = value.split(' ', 1)
  
  Above, the ``FirstNameLastName`` class refers to the hybrid from
  ``FirstNameOnly.name`` to repurpose its getter and setter for the subclass.
  
  When overriding :meth:`.hybrid_property.expression` and
  :meth:`.hybrid_property.comparator` alone as the first reference to the
  superclass, these names conflict with the same-named accessors on the class-
  level :class:`.QueryableAttribute` object returned at the class level.  To
  override these methods when referring directly to the parent class descriptor,
  add the special qualifier :attr:`.hybrid_property.overrides`, which will de-
  reference the instrumented attribute back to the hybrid object::
  
      class FirstNameLastName(FirstNameOnly):
          # ...
  
          last_name = Column(String)
  
          @FirstNameOnly.name.overrides.expression
          def name(cls):
              return func.concat(cls.first_name, ' ', cls.last_name)
  
  .. versionadded:: 1.2 Added :meth:`.hybrid_property.getter` as well as the
     ability to redefine accessors per-subclass.
  
  
  Hybrid Value Objects
  --------------------
  
  Note in our previous example, if we were to compare the ``word_insensitive``
  attribute of a ``SearchWord`` instance to a plain Python string, the plain
  Python string would not be coerced to lower case - the
  ``CaseInsensitiveComparator`` we built, being returned by
  ``@word_insensitive.comparator``, only applies to the SQL side.
  
  A more comprehensive form of the custom comparator is to construct a *Hybrid
  Value Object*. This technique applies the target value or expression to a value
  object which is then returned by the accessor in all cases.   The value object
  allows control of all operations upon the value as well as how compared values
  are treated, both on the SQL expression side as well as the Python value side.
  Replacing the previous ``CaseInsensitiveComparator`` class with a new
  ``CaseInsensitiveWord`` class::
  
      class CaseInsensitiveWord(Comparator):
          "Hybrid value representing a lower case representation of a word."
  
          def __init__(self, word):
              if isinstance(word, basestring):
                  self.word = word.lower()
              elif isinstance(word, CaseInsensitiveWord):
                  self.word = word.word
              else:
                  self.word = func.lower(word)
  
          def operate(self, op, other):
              if not isinstance(other, CaseInsensitiveWord):
                  other = CaseInsensitiveWord(other)
              return op(self.word, other.word)
  
          def __clause_element__(self):
              return self.word
  
          def __str__(self):
              return self.word
  
          key = 'word'
          "Label to apply to Query tuple results"
  
  Above, the ``CaseInsensitiveWord`` object represents ``self.word``, which may
  be a SQL function, or may be a Python native.   By overriding ``operate()`` and
  ``__clause_element__()`` to work in terms of ``self.word``, all comparison
  operations will work against the "converted" form of ``word``, whether it be
  SQL side or Python side. Our ``SearchWord`` class can now deliver the
  ``CaseInsensitiveWord`` object unconditionally from a single hybrid call::
  
      class SearchWord(Base):
          __tablename__ = 'searchword'
          id = Column(Integer, primary_key=True)
          word = Column(String(255), nullable=False)
  
          @hybrid_property
          def word_insensitive(self):
              return CaseInsensitiveWord(self.word)
  
  The ``word_insensitive`` attribute now has case-insensitive comparison behavior
  universally, including SQL expression vs. Python expression (note the Python
  value is converted to lower case on the Python side here)::
  
      >>> print(Session().query(SearchWord).filter_by(word_insensitive="Trucks"))
      SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
      FROM searchword
      WHERE lower(searchword.word) = :lower_1
  
  SQL expression versus SQL expression::
  
      >>> sw1 = aliased(SearchWord)
      >>> sw2 = aliased(SearchWord)
      >>> print(Session().query(
      ...                    sw1.word_insensitive,
      ...                    sw2.word_insensitive).\
      ...                        filter(
      ...                            sw1.word_insensitive > sw2.word_insensitive
      ...                        ))
      SELECT lower(searchword_1.word) AS lower_1,
      lower(searchword_2.word) AS lower_2
      FROM searchword AS searchword_1, searchword AS searchword_2
      WHERE lower(searchword_1.word) > lower(searchword_2.word)
  
  Python only expression::
  
      >>> ws1 = SearchWord(word="SomeWord")
      >>> ws1.word_insensitive == "sOmEwOrD"
      True
      >>> ws1.word_insensitive == "XOmEwOrX"
      False
      >>> print(ws1.word_insensitive)
      someword
  
  The Hybrid Value pattern is very useful for any kind of value that may have
  multiple representations, such as timestamps, time deltas, units of
  measurement, currencies and encrypted passwords.
  
  .. seealso::
  
      `Hybrids and Value Agnostic Types
      <http://techspot.zzzeek.org/2011/10/21/hybrids-and-value-agnostic-types/>`_
      - on the techspot.zzzeek.org blog
  
      `Value Agnostic Types, Part II
      <http://techspot.zzzeek.org/2011/10/29/value-agnostic-types-part-ii/>`_ -
      on the techspot.zzzeek.org blog
  
  .. _hybrid_transformers:
  
  Building Transformers
  ----------------------
  
  A *transformer* is an object which can receive a :class:`_query.Query`
  object and
  return a new one.   The :class:`_query.Query` object includes a method
  :meth:`.with_transformation` that returns a new :class:`_query.Query`
  transformed by
  the given function.
  
  We can combine this with the :class:`.Comparator` class to produce one type
  of recipe which can both set up the FROM clause of a query as well as assign
  filtering criterion.
  
  Consider a mapped class ``Node``, which assembles using adjacency list into a
  hierarchical tree pattern::
  
      from sqlalchemy import Column, Integer, ForeignKey
      from sqlalchemy.orm import relationship
      from sqlalchemy.ext.declarative import declarative_base
      Base = declarative_base()
  
      class Node(Base):
          __tablename__ = 'node'
          id = Column(Integer, primary_key=True)
          parent_id = Column(Integer, ForeignKey('node.id'))
          parent = relationship("Node", remote_side=id)
  
  Suppose we wanted to add an accessor ``grandparent``.  This would return the
  ``parent`` of ``Node.parent``.  When we have an instance of ``Node``, this is
  simple::
  
      from sqlalchemy.ext.hybrid import hybrid_property
  
      class Node(Base):
          # ...
  
          @hybrid_property
          def grandparent(self):
              return self.parent.parent
  
  For the expression, things are not so clear.   We'd need to construct a
  :class:`_query.Query` where we :meth:`_query.Query.join` twice along
  ``Node.parent`` to get to the ``grandparent``.   We can instead return a
  transforming callable that we'll combine with the :class:`.Comparator` class to
  receive any :class:`_query.Query` object, and return a new one that's joined to
  the ``Node.parent`` attribute and filtered based on the given criterion::
  
      from sqlalchemy.ext.hybrid import Comparator
  
      class GrandparentTransformer(Comparator):
          def operate(self, op, other):
              def transform(q):
                  cls = self.__clause_element__()
                  parent_alias = aliased(cls)
                  return q.join(parent_alias, cls.parent).\
                              filter(op(parent_alias.parent, other))
              return transform
  
      Base = declarative_base()
  
      class Node(Base):
          __tablename__ = 'node'
          id = Column(Integer, primary_key=True)
          parent_id = Column(Integer, ForeignKey('node.id'))
          parent = relationship("Node", remote_side=id)
  
          @hybrid_property
          def grandparent(self):
              return self.parent.parent
  
          @grandparent.comparator
          def grandparent(cls):
              return GrandparentTransformer(cls)
  
  The ``GrandparentTransformer`` overrides the core :meth:`.Operators.operate`
  method at the base of the :class:`.Comparator` hierarchy to return a query-
  transforming callable, which then runs the given comparison operation in a
  particular context. Such as, in the example above, the ``operate`` method is
  called, given the :attr:`.Operators.eq` callable as well as the right side of
  the comparison ``Node(id=5)``.  A function ``transform`` is then returned which
  will transform a :class:`_query.Query` first to join to ``Node.parent``,
  then to
  compare ``parent_alias`` using :attr:`.Operators.eq` against the left and right
  sides, passing into :meth:`_query.Query.filter`:
  
  .. sourcecode:: pycon+sql
  
      >>> from sqlalchemy.orm import Session
      >>> session = Session()
      {sql}>>> session.query(Node).\
      ...        with_transformation(Node.grandparent==Node(id=5)).\
      ...        all()
      SELECT node.id AS node_id, node.parent_id AS node_parent_id
      FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
      WHERE :param_1 = node_1.parent_id
      {stop}
  
  We can modify the pattern to be more verbose but flexible by separating the
  "join" step from the "filter" step.  The tricky part here is ensuring that
  successive instances of ``GrandparentTransformer`` use the same
  :class:`.AliasedClass` object against ``Node``.  Below we use a simple
  memoizing approach that associates a ``GrandparentTransformer`` with each
  class::
  
      class Node(Base):
  
          # ...
  
          @grandparent.comparator
          def grandparent(cls):
              # memoize a GrandparentTransformer
              # per class
              if '_gp' not in cls.__dict__:
                  cls._gp = GrandparentTransformer(cls)
              return cls._gp
  
      class GrandparentTransformer(Comparator):
  
          def __init__(self, cls):
              self.parent_alias = aliased(cls)
  
          @property
          def join(self):
              def go(q):
                  return q.join(self.parent_alias, Node.parent)
              return go
  
          def operate(self, op, other):
              return op(self.parent_alias.parent, other)
  
  .. sourcecode:: pycon+sql
  
      {sql}>>> session.query(Node).\
      ...            with_transformation(Node.grandparent.join).\
      ...            filter(Node.grandparent==Node(id=5))
      SELECT node.id AS node_id, node.parent_id AS node_parent_id
      FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
      WHERE :param_1 = node_1.parent_id
      {stop}
  
  The "transformer" pattern is an experimental pattern that starts to make usage
  of some functional programming paradigms. While it's only recommended for
  advanced and/or patient developers, there's probably a whole lot of amazing
  things it can be used for.
  
  """  # noqa
  from .. import util
  from ..orm import attributes
  from ..orm import interfaces
  
  
  HYBRID_METHOD = util.symbol("HYBRID_METHOD")
  """Symbol indicating an :class:`InspectionAttr` that's
     of type :class:`.hybrid_method`.
  
     Is assigned to the :attr:`.InspectionAttr.extension_type`
     attribute.
  
     .. seealso::
  
      :attr:`_orm.Mapper.all_orm_attributes`
  
  """
  
  HYBRID_PROPERTY = util.symbol("HYBRID_PROPERTY")
  """Symbol indicating an :class:`InspectionAttr` that's
      of type :class:`.hybrid_method`.
  
     Is assigned to the :attr:`.InspectionAttr.extension_type`
     attribute.
  
     .. seealso::
  
      :attr:`_orm.Mapper.all_orm_attributes`
  
  """
  
  
  class hybrid_method(interfaces.InspectionAttrInfo):
      """A decorator which allows definition of a Python object method with both
      instance-level and class-level behavior.
  
      """
  
      is_attribute = True
      extension_type = HYBRID_METHOD
  
      def __init__(self, func, expr=None):
          """Create a new :class:`.hybrid_method`.
  
          Usage is typically via decorator::
  
              from sqlalchemy.ext.hybrid import hybrid_method
  
              class SomeClass(object):
                  @hybrid_method
                  def value(self, x, y):
                      return self._value + x + y
  
                  @value.expression
                  def value(self, x, y):
                      return func.some_function(self._value, x, y)
  
          """
          self.func = func
          self.expression(expr or func)
  
      def __get__(self, instance, owner):
          if instance is None:
              return self.expr.__get__(owner, owner.__class__)
          else:
              return self.func.__get__(instance, owner)
  
      def expression(self, expr):
          """Provide a modifying decorator that defines a
          SQL-expression producing method."""
  
          self.expr = expr
          if not self.expr.__doc__:
              self.expr.__doc__ = self.func.__doc__
          return self
  
  
  class hybrid_property(interfaces.InspectionAttrInfo):
      """A decorator which allows definition of a Python descriptor with both
      instance-level and class-level behavior.
  
      """
  
      is_attribute = True
      extension_type = HYBRID_PROPERTY
  
      def __init__(
          self,
          fget,
          fset=None,
          fdel=None,
          expr=None,
          custom_comparator=None,
          update_expr=None,
      ):
          """Create a new :class:`.hybrid_property`.
  
          Usage is typically via decorator::
  
              from sqlalchemy.ext.hybrid import hybrid_property
  
              class SomeClass(object):
                  @hybrid_property
                  def value(self):
                      return self._value
  
                  @value.setter
                  def value(self, value):
                      self._value = value
  
          """
          self.fget = fget
          self.fset = fset
          self.fdel = fdel
          self.expr = expr
          self.custom_comparator = custom_comparator
          self.update_expr = update_expr
          util.update_wrapper(self, fget)
  
      def __get__(self, instance, owner):
          if instance is None:
              return self._expr_comparator(owner)
          else:
              return self.fget(instance)
  
      def __set__(self, instance, value):
          if self.fset is None:
              raise AttributeError("can't set attribute")
          self.fset(instance, value)
  
      def __delete__(self, instance):
          if self.fdel is None:
              raise AttributeError("can't delete attribute")
          self.fdel(instance)
  
      def _copy(self, **kw):
          defaults = {
              key: value
              for key, value in self.__dict__.items()
              if not key.startswith("_")
          }
          defaults.update(**kw)
          return type(self)(**defaults)
  
      @property
      def overrides(self):
          """Prefix for a method that is overriding an existing attribute.
  
          The :attr:`.hybrid_property.overrides` accessor just returns
          this hybrid object, which when called at the class level from
          a parent class, will de-reference the "instrumented attribute"
          normally returned at this level, and allow modifying decorators
          like :meth:`.hybrid_property.expression` and
          :meth:`.hybrid_property.comparator`
          to be used without conflicting with the same-named attributes
          normally present on the :class:`.QueryableAttribute`::
  
              class SuperClass(object):
                  # ...
  
                  @hybrid_property
                  def foobar(self):
                      return self._foobar
  
              class SubClass(SuperClass):
                  # ...
  
                  @SuperClass.foobar.overrides.expression
                  def foobar(cls):
                      return func.subfoobar(self._foobar)
  
          .. versionadded:: 1.2
  
          .. seealso::
  
              :ref:`hybrid_reuse_subclass`
  
          """
          return self
  
      def getter(self, fget):
          """Provide a modifying decorator that defines a getter method.
  
          .. versionadded:: 1.2
  
          """
  
          return self._copy(fget=fget)
  
      def setter(self, fset):
          """Provide a modifying decorator that defines a setter method."""
  
          return self._copy(fset=fset)
  
      def deleter(self, fdel):
          """Provide a modifying decorator that defines a deletion method."""
  
          return self._copy(fdel=fdel)
  
      def expression(self, expr):
          """Provide a modifying decorator that defines a SQL-expression
          producing method.
  
          When a hybrid is invoked at the class level, the SQL expression given
          here is wrapped inside of a specialized :class:`.QueryableAttribute`,
          which is the same kind of object used by the ORM to represent other
          mapped attributes.   The reason for this is so that other class-level
          attributes such as docstrings and a reference to the hybrid itself may
          be maintained within the structure that's returned, without any
          modifications to the original SQL expression passed in.
  
          .. note::
  
             When referring to a hybrid property  from an owning class (e.g.
             ``SomeClass.some_hybrid``), an instance of
             :class:`.QueryableAttribute` is returned, representing the
             expression or comparator object as well as this  hybrid object.
             However, that object itself has accessors called ``expression`` and
             ``comparator``; so when attempting to override these decorators on a
             subclass, it may be necessary to qualify it using the
             :attr:`.hybrid_property.overrides` modifier first.  See that
             modifier for details.
  
          .. seealso::
  
              :ref:`hybrid_distinct_expression`
  
          """
  
          return self._copy(expr=expr)
  
      def comparator(self, comparator):
          """Provide a modifying decorator that defines a custom
          comparator producing method.
  
          The return value of the decorated method should be an instance of
          :class:`~.hybrid.Comparator`.
  
          .. note::  The :meth:`.hybrid_property.comparator` decorator
             **replaces** the use of the :meth:`.hybrid_property.expression`
             decorator.  They cannot be used together.
  
          When a hybrid is invoked at the class level, the
          :class:`~.hybrid.Comparator` object given here is wrapped inside of a
          specialized :class:`.QueryableAttribute`, which is the same kind of
          object used by the ORM to represent other mapped attributes.   The
          reason for this is so that other class-level attributes such as
          docstrings and a reference to the hybrid itself may be maintained
          within the structure that's returned, without any modifications to the
          original comparator object passed in.
  
          .. note::
  
             When referring to a hybrid property  from an owning class (e.g.
             ``SomeClass.some_hybrid``), an instance of
             :class:`.QueryableAttribute` is returned, representing the
             expression or comparator object as this  hybrid object.  However,
             that object itself has accessors called ``expression`` and
             ``comparator``; so when attempting to override these decorators on a
             subclass, it may be necessary to qualify it using the
             :attr:`.hybrid_property.overrides` modifier first.  See that
             modifier for details.
  
          """
          return self._copy(custom_comparator=comparator)
  
      def update_expression(self, meth):
          """Provide a modifying decorator that defines an UPDATE tuple
          producing method.
  
          The method accepts a single value, which is the value to be
          rendered into the SET clause of an UPDATE statement.  The method
          should then process this value into individual column expressions
          that fit into the ultimate SET clause, and return them as a
          sequence of 2-tuples.  Each tuple
          contains a column expression as the key and a value to be rendered.
  
          E.g.::
  
              class Person(Base):
                  # ...
  
                  first_name = Column(String)
                  last_name = Column(String)
  
                  @hybrid_property
                  def fullname(self):
                      return first_name + " " + last_name
  
                  @fullname.update_expression
                  def fullname(cls, value):
                      fname, lname = value.split(" ", 1)
                      return [
                          (cls.first_name, fname),
                          (cls.last_name, lname)
                      ]
  
          .. versionadded:: 1.2
  
          """
          return self._copy(update_expr=meth)
  
      @util.memoized_property
      def _expr_comparator(self):
          if self.custom_comparator is not None:
              return self._get_comparator(self.custom_comparator)
          elif self.expr is not None:
              return self._get_expr(self.expr)
          else:
              return self._get_expr(self.fget)
  
      def _get_expr(self, expr):
          def _expr(cls):
              return ExprComparator(cls, expr(cls), self)
  
          util.update_wrapper(_expr, expr)
  
          return self._get_comparator(_expr)
  
      def _get_comparator(self, comparator):
  
          proxy_attr = attributes.create_proxied_attribute(self)
  
          def expr_comparator(owner):
              return proxy_attr(
                  owner,
                  self.__name__,
                  self,
                  comparator(owner),
                  doc=comparator.__doc__ or self.__doc__,
              )
  
          return expr_comparator
  
  
  class Comparator(interfaces.PropComparator):
      """A helper class that allows easy construction of custom
      :class:`~.orm.interfaces.PropComparator`
      classes for usage with hybrids."""
  
      property = None
  
      def __init__(self, expression):
          self.expression = expression
  
      def __clause_element__(self):
          expr = self.expression
          if hasattr(expr, "__clause_element__"):
              expr = expr.__clause_element__()
          return expr
  
      def adapt_to_entity(self, adapt_to_entity):
          # interesting....
          return self
  
  
  class ExprComparator(Comparator):
      def __init__(self, cls, expression, hybrid):
          self.cls = cls
          self.expression = expression
          self.hybrid = hybrid
  
      def __getattr__(self, key):
          return getattr(self.expression, key)
  
      @property
      def info(self):
          return self.hybrid.info
  
      def _bulk_update_tuples(self, value):
          if isinstance(self.expression, attributes.QueryableAttribute):
              return self.expression._bulk_update_tuples(value)
          elif self.hybrid.update_expr is not None:
              return self.hybrid.update_expr(self.cls, value)
          else:
              return [(self.expression, value)]
  
      @property
      def property(self):
          return self.expression.property
  
      def operate(self, op, *other, **kwargs):
          return op(self.expression, *other, **kwargs)
  
      def reverse_operate(self, op, other, **kwargs):
          return op(other, self.expression, **kwargs)
  