Encapsulates a set of objects being operated upon within an object-relational operation.
The Session is the front end to SQLAlchemy's Unit of Work implementation. The concept behind Unit of Work is to track modifications to a field of objects, and then be able to flush those changes to the database in a single operation.
SQLAlchemy's unit of work includes these functions:
When dealing with instances of mapped classes, an instance may be attached to a particular Session, else it is unattached . An instance also may or may not correspond to an actual row in the database. These conditions break up into four distinct states:
The session methods which control instance state include save(), update(), save_or_update(), delete(), merge(), and expunge().
The Session object is not threadsafe, particularly during flush operations. A session which is only read from (i.e. is never flushed) can be used by concurrent threads if it's acceptable that some object instances may be loaded twice.
The typical pattern to managing Sessions in a multi-threaded environment is either to use mutexes to limit concurrent access to one thread at a time, or more commonly to establish a unique session for every thread, using a threadlocal variable. SQLAlchemy provides a thread-managed Session adapter, provided by the scoped_session() function.
Construct a new Session.
an optional dictionary, which contains more granular "bind" information than the bind parameter provides. This dictionary can map individual Table instances as well as Mapper instances to individual Engine or Connection objects. Operations which proceed relative to a particular Mapper will consult this dictionary for the direct Mapper instance as well as the mapper's mapped_table attribute in order to locate an connectable to use. The full resolution is described in the get_bind() method of Session. Usage looks like:
sess = Session(binds={ SomeMappedClass : create_engine('postgres://engine1'), somemapper : create_engine('postgres://engine2'), some_table : create_engine('postgres://engine3'), })
Also see the bind_mapper() and bind_table() methods.
begin a 'nested' transaction on this Session.
this utilizes a SAVEPOINT transaction for databases which support this feature.
Bind the given mapper or class to the given Engine or Connection.
All subsequent operations involving this Mapper will use the given bind.
Bind the given table to the given Engine or Connection.
All subsequent operations involving this Table will use the given bind.
Remove all object instances from this Session.
This is equivalent to calling expunge() for all objects in this Session.
Close this Session.
This clears all items and ends any transaction in progress.
If this session were created with transactional=True, a new transaction is immediately begun. Note that this new transaction does not use any connection resources until they are first needed.
commit the current transaction in progress.
If no transaction is in progress, this method raises an InvalidRequestError.
If the begin() method was called on this Session additional times subsequent to its first call, commit() will not actually commit, and instead pops an internal SessionTransaction off its internal stack of transactions. Only when the "root" SessionTransaction is reached does an actual database-level commit occur.
Return a Connection corresponding to this session's transactional context, if any.
If this Session is transactional, the connection will be in the context of this session's transaction. Otherwise, the connection is returned by the contextual_connect() method, which some Engines override to return a thread-local connection, and will have close_with_result set to True.
The given **kwargs will be sent to the engine's contextual_connect() method, if no transaction is in progress.
the "mapper" argument is a class or mapper to which a bound engine will be located; use this when the Session itself is either bound to multiple engines or connections, or is not bound to any connectable.
Mark the given instance as deleted.
The delete operation occurs upon flush().
Using the given mapper to identify the appropriate Engine or Connection to be used for statement execution, execute the given ClauseElement using the provided parameter dictionary.
Return a ResultProxy corresponding to the execution's results.
If this method allocates a new Connection for the operation, then the ResultProxy 's close() method will release the resources of the underlying Connection.
Mark the given object as expired.
This will add an instrumentation to all mapped attributes on the instance such that when an attribute is next accessed, the session will reload all attributes on the instance from the database.
Remove the given object from this Session.
This will free all internal references to the object. Cascading will be applied according to the expunge cascade rule.
Flush all the object modifications present in this session to the database.
objects is a list or tuple of objects specifically to be flushed; if None, all new and modified objects are flushed.
Return an instance of the object based on the given identifier, or None if not found.
The ident argument is a scalar or tuple of primary key column values in the order of the table def's primary key columns.
The entity_name keyword argument may also be specified which further qualifies the underlying Mapper used to perform the query.
Get an identity key.
Valid call signatures:
identity_key(class, ident, entity_name=None)
mapped class (must be a positional argument)
primary key, if the key is composite this is a tuple
optional entity name
identity_key(instance=instance)
object instance (must be given as a keyword arg)
identity_key(class, row=row, entity_name=None)
mapped class (must be a positional argument)
result proxy row (must be given as a keyword arg)
optional entity name (must be given as a keyword arg)
Return True if the given object has been marked as expired.
Return an instance of the object based on the given identifier.
If not found, raises an exception. The method will remove all pending changes to the object already existing in the Session. The ident argument is a scalar or tuple of primary key columns in the order of the table def's primary key columns.
The entity_name keyword argument may also be specified which further qualifies the underlying Mapper used to perform the query.
Copy the state of the given object onto the persistent object with the same identifier.
If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance. The given instance does not become associated with the session.
This operation cascades to associated instances if the association is mapped with cascade="merge".
Removes unreferenced instances cached in the identity map.
Removes any object in this Session'sidentity map that is not referenced in user code, modified, new or scheduled for deletion. Returns the number of objects pruned.
Return a new Query object corresponding to this Session and the mapper, or the classes' primary mapper.
Reload the attributes for the given object from the database, clear any changes made.
rollback the current transaction in progress.
If no transaction is in progress, this method is a pass-thru.
Add a transient (unsaved) instance to this Session.
This operation cascades the save_or_update method to associated instances if the relation is mapped with cascade="save-update".
The entity_name keyword argument will further qualify the specific Mapper used to handle this instance.
Save or update the given object into this Session.
The presence of an _instance_key attribute on the instance determines whether to save() or update() the instance.
Like execute() but return a scalar result.
Bring the given detached (saved) instance into this Session.
If there is a persistent instance with the same identifier already associated with this Session, an exception is thrown.
This operation cascades the save_or_update method to associated instances if the relation is mapped with cascade="save-update".
return True if the given object is associated with this session.
The instance may be pending or persistent within the Session for a result of True.
return an iterator of all objects which are pending or persistent within this Session.