sqlalchemy.ext.horizontal_shard.ShardedSession

Construct a ShardedSession. :param shard_chooser: A callable which, passed a Mapper, a mapped instance, and possibly a SQL clause, returns a shard ID. This id may be based off of the attributes present within the object, or on some round-robin scheme. If the scheme is based on a selection, it should set whatever state on the instance to mark it in the future as participating in that shard. :param identity_chooser: A callable, passed a Mapper and primary key argument, which should return a list of shard ids where this primary key might reside. .. versionchanged:: 2.0 The ``identity_chooser`` parameter supersedes the ``id_chooser`` parameter. :param execute_chooser: For a given :class:`.ORMExecuteState`, returns the list of shard_ids where the query should be issued. Results from all shards returned will be combined together into a single listing. .. versionchanged:: 1.4 The ``execute_chooser`` parameter supersedes the ``query_chooser`` parameter. :param shards: A dictionary of string shard names to :class:`~sqlalchemy.engine.Engine` objects.

Undocumented

Provide a :class:`_engine.Connection` to use in the unit of work flush process.

Return a "bind" to which this :class:`.Session` is bound. The "bind" is usually an instance of :class:`_engine.Engine`, except in the case where the :class:`.Session` has been explicitly bound directly to a :class:`_engine.Connection`. For a multiply-bound or unbound :class:`.Session`, the ``mapper`` or ``clause`` arguments are used to determine the appropriate bind to return. Note that the "mapper" argument is usually present when :meth:`.Session.get_bind` is called via an ORM operation such as a :meth:`.Session.query`, each individual INSERT/UPDATE/DELETE operation within a :meth:`.Session.flush`, call, etc. The order of resolution is: 1. if mapper given and :paramref:`.Session.binds` is present, locate a bind based first on the mapper in use, then on the mapped class in use, then on any base classes that are present in the ``__mro__`` of the mapped class, from more specific superclasses to more general. 2. if clause given and ``Session.binds`` is present, locate a bind based on :class:`_schema.Table` objects found in the given clause present in ``Session.binds``. 3. if ``Session.binds`` is present, return that. 4. if clause given, attempt to return a bind linked to the :class:`_schema.MetaData` ultimately associated with the clause. 5. if mapper given, attempt to return a bind linked to the :class:`_schema.MetaData` ultimately associated with the :class:`_schema.Table` or other selectable to which the mapper is mapped. 6. No bind can be found, :exc:`~sqlalchemy.exc.UnboundExecutionError` is raised. Note that the :meth:`.Session.get_bind` method can be overridden on a user-defined subclass of :class:`.Session` to provide any kind of bind resolution scheme. See the example at :ref:`session_custom_partitioning`. :param mapper: Optional mapped class or corresponding :class:`_orm.Mapper` instance. The bind can be derived from a :class:`_orm.Mapper` first by consulting the "binds" map associated with this :class:`.Session`, and secondly by consulting the :class:`_schema.MetaData` associated with the :class:`_schema.Table` to which the :class:`_orm.Mapper` is mapped for a bind. :param clause: A :class:`_expression.ClauseElement` (i.e. :func:`_expression.select`, :func:`_expression.text`, etc.). If the ``mapper`` argument is not present or could not produce a bind, the given expression construct will be searched for a bound element, typically a :class:`_schema.Table` associated with bound :class:`_schema.MetaData`. .. seealso:: :ref:`session_partitioning` :paramref:`.Session.binds` :meth:`.Session.bind_mapper` :meth:`.Session.bind_table`