Class ForeignKey

lib/sqlalchemy/sql/schema.py:3032–3628

Defines a dependency between two columns. ``ForeignKey`` is specified as an argument to a :class:`_schema.Column` object, e.g.:: t = Table( "remote_table", metadata, Column("remote_id", ForeignKey("main_table.id")), ) Note that `

class ForeignKey(DialectKWArgs, SchemaItem):
    """Defines a dependency between two columns.

    ``ForeignKey`` is specified as an argument to a :class:`_schema.Column`
    object,
    e.g.::

        t = Table(
            "remote_table",
            metadata,
            Column("remote_id", ForeignKey("main_table.id")),
        )

    Note that ``ForeignKey`` is only a marker object that defines
    a dependency between two columns.   The actual constraint
    is in all cases represented by the :class:`_schema.ForeignKeyConstraint`
    object.   This object will be generated automatically when
    a ``ForeignKey`` is associated with a :class:`_schema.Column` which
    in turn is associated with a :class:`_schema.Table`.   Conversely,
    when :class:`_schema.ForeignKeyConstraint` is applied to a
    :class:`_schema.Table`,
    ``ForeignKey`` markers are automatically generated to be
    present on each associated :class:`_schema.Column`, which are also
    associated with the constraint object.

    Note that you cannot define a "composite" foreign key constraint,
    that is a constraint between a grouping of multiple parent/child
    columns, using ``ForeignKey`` objects.   To define this grouping,
    the :class:`_schema.ForeignKeyConstraint` object must be used, and applied
    to the :class:`_schema.Table`.   The associated ``ForeignKey`` objects
    are created automatically.

    The ``ForeignKey`` objects associated with an individual
    :class:`_schema.Column`
    object are available in the `foreign_keys` collection
    of that column.

    Further examples of foreign key configuration are in
    :ref:`metadata_foreignkeys`.

    """

    __visit_name__ = "foreign_key"

    parent: Column[Any]

    _table_column: Optional[Column[Any]]

    _colspec: Union[str, Column[Any]]

    def __init__(
        self,
        column: _DDLColumnReferenceArgument,
        _constraint: Optional[ForeignKeyConstraint] = None,
        use_alter: bool = False,
        name: _ConstraintNameArgument = None,
        onupdate: Optional[str] = None,
        ondelete: Optional[str] = None,