# ext/hybrid.py
# Copyright (C) 2005-2018 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
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 :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

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.update` method, allowing the hybrid to be used in the
SET clause of the update.

Normally, when using a hybrid with :meth:`.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.update` which can affect
this, using the :meth:`.hybrid_propery.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)