text: # ext/hybrid.py
  # Copyright (C) 2005-2014 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
  
  """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
  returns a new SQL expression::
  
      >>> print Interval.length
      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.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"
  
  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 :attr:`.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
  
  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
  
  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.  Two variants of 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.
  
  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 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`
  object and return a new one.   The :class:`.Query` object includes a
  method :meth:`.with_transformation` that returns a new :class:`.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` where we :meth:`~.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` 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`
  first to join to ``Node.parent``, then to compare ``parent_alias``
  using :attr:`.Operators.eq` against the left and right sides, passing
  into :class:`.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.
  
  """
  from .. import util
  from ..orm import attributes, 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`
     attibute.
  
     .. seealso::
  
      :attr:`.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`
     attibute.
  
     .. seealso::
  
      :attr:`.Mapper.all_orm_attributes`
  
  """
  
  class hybrid_method(interfaces._InspectionAttr):
      """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.expr = 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
          return self
  
  
  class hybrid_property(interfaces._InspectionAttr):
      """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):
          """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 or fget
          util.update_wrapper(self, fget)
  
      def __get__(self, instance, owner):
          if instance is None:
              return self.expr(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 setter(self, fset):
          """Provide a modifying decorator that defines a value-setter method."""
  
          self.fset = fset
          return self
  
      def deleter(self, fdel):
          """Provide a modifying decorator that defines a
          value-deletion method."""
  
          self.fdel = fdel
          return self
  
      def expression(self, expr):
          """Provide a modifying decorator that defines a SQL-expression
          producing method."""
  
          self.expr = expr
          return self
  
      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`.
  
          """
  
          proxy_attr = attributes.\
                          create_proxied_attribute(self)
  
          def expr(owner):
              return proxy_attr(owner, self.__name__, self, comparator(owner))
          self.expr = expr
          return self
  
  
  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
          while hasattr(expr, '__clause_element__'):
              expr = expr.__clause_element__()
          return expr
  
      def adapt_to_entity(self, adapt_to_entity):
          # interesting....
          return self
  