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Classe « FunctionElement »

Méthode sqlalchemy.FunctionElement.as_comparison

Signature de la méthode as_comparison

def as_comparison(self, left_index: 'int', right_index: 'int') -> 'FunctionAsBinary' 

Description

help(FunctionElement.as_comparison)

Interpret this expression as a boolean comparison between two
values.

This method is used for an ORM use case described at
:ref:`relationship_custom_operator_sql_function`.

A hypothetical SQL function "is_equal()" which compares to values
for equality would be written in the Core expression language as::

    expr = func.is_equal("a", "b")

If "is_equal()" above is comparing "a" and "b" for equality, the
:meth:`.FunctionElement.as_comparison` method would be invoked as::

    expr = func.is_equal("a", "b").as_comparison(1, 2)

Where above, the integer value "1" refers to the first argument of the
"is_equal()" function and the integer value "2" refers to the second.

This would create a :class:`.BinaryExpression` that is equivalent to::

    BinaryExpression("a", "b", operator=op.eq)

However, at the SQL level it would still render as
"is_equal('a', 'b')".

The ORM, when it loads a related object or collection, needs to be able
to manipulate the "left" and "right" sides of the ON clause of a JOIN
expression. The purpose of this method is to provide a SQL function
construct that can also supply this information to the ORM, when used
with the :paramref:`_orm.relationship.primaryjoin` parameter. The
return value is a containment object called :class:`.FunctionAsBinary`.

An ORM example is as follows::

    class Venue(Base):
        __tablename__ = "venue"
        id = Column(Integer, primary_key=True)
        name = Column(String)

        descendants = relationship(
            "Venue",
            primaryjoin=func.instr(
                remote(foreign(name)), name + "/"
            ).as_comparison(1, 2)
            == 1,
            viewonly=True,
            order_by=name,
        )

Above, the "Venue" class can load descendant "Venue" objects by
determining if the name of the parent Venue is contained within the
start of the hypothetical descendant value's name, e.g. "parent1" would
match up to "parent1/child1", but not to "parent2/child1".

Possible use cases include the "materialized path" example given above,
as well as making use of special SQL functions such as geometric
functions to create join conditions.

:param left_index: the integer 1-based index of the function argument
 that serves as the "left" side of the expression.
:param right_index: the integer 1-based index of the function argument
 that serves as the "right" side of the expression.

.. versionadded:: 1.3

.. seealso::

    :ref:`relationship_custom_operator_sql_function` -
    example use within the ORM

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