Session Basics¶
What does the Session do ?¶
In the most general sense, the Session establishes all conversations
with the database and represents a “holding zone” for all the objects which
you’ve loaded or associated with it during its lifespan. It provides the
interface where SELECT and other queries are made that will return and modify
ORM-mapped objects. The ORM objects themselves are maintained inside the
Session, inside a structure called the identity map - a data
structure that maintains unique copies of each object, where “unique” means
“only one object with a particular primary key”.
The Session begins in a mostly stateless form. Once queries are
issued or other objects are persisted with it, it requests a connection
resource from an Engine that is associated with the
Session, and then establishes a transaction on that connection. This
transaction remains in effect until the Session is instructed to
commit or roll back the transaction.
The ORM objects maintained by a Session are instrumented
such that whenever an attribute or a collection is modified in the Python
program, a change event is generated which is recorded by the
Session. Whenever the database is about to be queried, or when
the transaction is about to be committed, the Session first
flushes all pending changes stored in memory to the database. This is
known as the unit of work pattern.
When using a Session, it’s useful to consider the ORM mapped objects
that it maintains as proxy objects to database rows, which are local to the
transaction being held by the Session. In order to maintain the
state on the objects as matching what’s actually in the database, there are a
variety of events that will cause objects to re-access the database in order to
keep synchronized. It is possible to “detach” objects from a
Session, and to continue using them, though this practice has its
caveats. It’s intended that usually, you’d re-associate detached objects with
another Session when you want to work with them again, so that they
can resume their normal task of representing database state.
Basics of Using a Session¶
The most basic Session use patterns are presented here.
Opening and Closing a Session¶
The Session may be constructed on its own or by using the
sessionmaker class. It typically is passed a single
Engine as a source of connectivity up front. A typical use
may look like:
from sqlalchemy import create_engine
from sqlalchemy.orm import Session
# an Engine, which the Session will use for connection
# resources
engine = create_engine("postgresql+psycopg2://scott:tiger@localhost/")
# create session and add objects
with Session(engine) as session:
session.add(some_object)
session.add(some_other_object)
session.commit()Above, the Session is instantiated with an Engine
associated with a particular database URL. It is then used in a Python
context manager (i.e. with: statement) so that it is automatically
closed at the end of the block; this is equivalent
to calling the Session.close() method.
The call to Session.commit() is optional, and is only needed if the
work we’ve done with the Session includes new data to be
persisted to the database. If we were only issuing SELECT calls and did not
need to write any changes, then the call to Session.commit() would
be unnecessary.
Note
Note that after Session.commit() is called, either explicitly or
when using a context manager, all objects associated with the
Session are expired, meaning their contents are erased to
be re-loaded within the next transaction. If these objects are instead
detached, they will be non-functional until re-associated with a
new Session, unless the Session.expire_on_commit
parameter is used to disable this behavior. See the
section Committing for more detail.
Framing out a begin / commit / rollback block¶
We may also enclose the Session.commit() call and the overall
“framing” of the transaction within a context manager for those cases where
we will be committing data to the database. By “framing” we mean that if all
operations succeed, the Session.commit() method will be called,
but if any exceptions are raised, the Session.rollback() method
will be called so that the transaction is rolled back immediately, before
propagating the exception outward. In Python this is most fundamentally
expressed using a try: / except: / else: block such as:
# verbose version of what a context manager will do
with Session(engine) as session:
session.begin()
try:
session.add(some_object)
session.add(some_other_object)
except:
session.rollback()
raise
else:
session.commit()The long-form sequence of operations illustrated above can be
achieved more succinctly by making use of the
SessionTransaction object returned by the Session.begin()
method, which provides a context manager interface for the same sequence of
operations:
# create session and add objects
with Session(engine) as session:
with session.begin():
session.add(some_object)
session.add(some_other_object)
# inner context calls session.commit(), if there were no exceptions
# outer context calls session.close()More succinctly, the two contexts may be combined:
# create session and add objects
with Session(engine) as session, session.begin():
session.add(some_object)
session.add(some_other_object)
# inner context calls session.commit(), if there were no exceptions
# outer context calls session.close()Using a sessionmaker¶
The purpose of sessionmaker is to provide a factory for
Session objects with a fixed configuration. As it is typical
that an application will have an Engine object in module
scope, the sessionmaker can provide a factory for
Session objects that are against this engine:
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
# an Engine, which the Session will use for connection
# resources, typically in module scope
engine = create_engine("postgresql+psycopg2://scott:tiger@localhost/")
# a sessionmaker(), also in the same scope as the engine
Session = sessionmaker(engine)
# we can now construct a Session() without needing to pass the
# engine each time
with Session() as session:
session.add(some_object)
session.add(some_other_object)
session.commit()
# closes the sessionThe sessionmaker is analogous to the Engine
as a module-level factory for function-level sessions / connections. As such
it also has its own sessionmaker.begin() method, analogous
to Engine.begin(), which returns a Session object
and also maintains a begin/commit/rollback block:
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
# an Engine, which the Session will use for connection
# resources
engine = create_engine("postgresql+psycopg2://scott:tiger@localhost/")
# a sessionmaker(), also in the same scope as the engine
Session = sessionmaker(engine)
# we can now construct a Session() and include begin()/commit()/rollback()
# at once
with Session.begin() as session:
session.add(some_object)
session.add(some_other_object)
# commits the transaction, closes the sessionWhere above, the Session will both have its transaction committed
as well as that the Session will be closed, when the above
with: block ends.
When you write your application, the
sessionmaker factory should be scoped the same as the
Engine object created by create_engine(), which
is typically at module-level or global scope. As these objects are both
factories, they can be used by any number of functions and threads
simultaneously.
Querying¶
The primary means of querying is to make use of the select()
construct to create a Select object, which is then executed to
return a result using methods such as Session.execute() and
Session.scalars(). Results are then returned in terms of
Result objects, including sub-variants such as
ScalarResult.
A complete guide to SQLAlchemy ORM querying can be found at ORM Querying Guide. Some brief examples follow:
from sqlalchemy import select
from sqlalchemy.orm import Session
with Session(engine) as session:
# query for ``User`` objects
statement = select(User).filter_by(name="ed")
# list of ``User`` objects
user_obj = session.scalars(statement).all()
# query for individual columns
statement = select(User.name, User.fullname)
# list of Row objects
rows = session.execute(statement).all()Changed in version 2.0: “2.0” style querying is now standard. See 2.0 Migration - ORM Usage for migration notes from the 1.x series.
See also
Adding New or Existing Items¶
Session.add() is used to place instances in the
session. For transient (i.e. brand new) instances, this will have the effect
of an INSERT taking place for those instances upon the next flush. For
instances which are persistent (i.e. were loaded by this session), they are
already present and do not need to be added. Instances which are detached
(i.e. have been removed from a session) may be re-associated with a session
using this method:
user1 = User(name="user1")
user2 = User(name="user2")
session.add(user1)
session.add(user2)
session.commit() # write changes to the databaseTo add a list of items to the session at once, use
Session.add_all():
session.add_all([item1, item2, item3])The Session.add() operation cascades along
the save-update cascade. For more details see the section
Cascades.
Deleting¶
The Session.delete() method places an instance
into the Session’s list of objects to be marked as deleted:
# mark two objects to be deleted
session.delete(obj1)
session.delete(obj2)
# commit (or flush)
session.commit()Session.delete() marks an object for deletion, which will
result in a DELETE statement emitted for each primary key affected.
Before the pending deletes are flushed, objects marked by “delete” are present
in the Session.deleted collection. After the DELETE, they
are expunged from the Session, which becomes permanent after
the transaction is committed.
There are various important behaviors related to the
Session.delete() operation, particularly in how relationships to
other objects and collections are handled. There’s more information on how
this works in the section Cascades, but in general
the rules are:
Rows that correspond to mapped objects that are related to a deleted object via the
relationship()directive are not deleted by default. If those objects have a foreign key constraint back to the row being deleted, those columns are set to NULL. This will cause a constraint violation if the columns are non-nullable.To change the “SET NULL” into a DELETE of a related object’s row, use the delete cascade on the
relationship().Rows that are in tables linked as “many-to-many” tables, via the
relationship.secondaryparameter, are deleted in all cases when the object they refer to is deleted.When related objects include a foreign key constraint back to the object being deleted, and the related collections to which they belong are not currently loaded into memory, the unit of work will emit a SELECT to fetch all related rows, so that their primary key values can be used to emit either UPDATE or DELETE statements on those related rows. In this way, the ORM without further instruction will perform the function of ON DELETE CASCADE, even if this is configured on Core
ForeignKeyConstraintobjects.The
relationship.passive_deletesparameter can be used to tune this behavior and rely upon “ON DELETE CASCADE” more naturally; when set to True, this SELECT operation will no longer take place, however rows that are locally present will still be subject to explicit SET NULL or DELETE. Settingrelationship.passive_deletesto the string"all"will disable all related object update/delete.When the DELETE occurs for an object marked for deletion, the object is not automatically removed from collections or object references that refer to it. When the
Sessionis expired, these collections may be loaded again so that the object is no longer present. However, it is preferable that instead of usingSession.delete()for these objects, the object should instead be removed from its collection and then delete-orphan should be used so that it is deleted as a secondary effect of that collection removal. See the section Notes on Delete - Deleting Objects Referenced from Collections and Scalar Relationships for an example of this.
See also
delete - describes “delete cascade”, which marks related objects for deletion when a lead object is deleted.
delete-orphan - describes “delete orphan cascade”, which marks related objects for deletion when they are de-associated from their lead object.
Notes on Delete - Deleting Objects Referenced from Collections and Scalar Relationships - important background on
Session.delete() as involves relationships being refreshed
in memory.
Flushing¶
When the Session is used with its default
configuration, the flush step is nearly always done transparently.
Specifically, the flush occurs before any individual
SQL statement is issued as a result of a Query or
a 2.0-style Session.execute() call, as well as within the
Session.commit() call before the transaction is
committed. It also occurs before a SAVEPOINT is issued when
Session.begin_nested() is used.
A Session flush can be forced at any time by calling the
Session.flush() method:
session.flush()The flush which occurs automatically within the scope of certain methods is known as autoflush. Autoflush is defined as a configurable, automatic flush call which occurs at the beginning of methods including:
Session.execute()and other SQL-executing methods, when used against ORM-enabled SQL constructs, such asselect()objects that refer to ORM entities and/or ORM-mapped attributesWhen a
Queryis invoked to send SQL to the databaseWithin the
Session.merge()method before querying the databaseWhen objects are refreshed
When ORM lazy load operations occur against unloaded object attributes.
There are also points at which flushes occur unconditionally; these points are within key transactional boundaries which include:
Within the process of the
Session.commit()methodWhen
Session.begin_nested()is calledWhen the
Session.prepare()2PC method is used.
The autoflush behavior, as applied to the previous list of items,
can be disabled by constructing a Session or
sessionmaker passing the Session.autoflush parameter as
False:
Session = sessionmaker(autoflush=False)Additionally, autoflush can be temporarily disabled within the flow
of using a Session using the
Session.no_autoflush context manager:
with mysession.no_autoflush:
mysession.add(some_object)
mysession.flush()To reiterate: The flush process always occurs when transactional
methods such as Session.commit() and Session.begin_nested() are
called, regardless of any “autoflush” settings, when the Session has
remaining pending changes to process.
As the Session only invokes SQL to the database within the context of
a DBAPI transaction, all “flush” operations themselves only occur within a
database transaction (subject to the
isolation level of the database
transaction), provided that the DBAPI is not in
driver level autocommit mode. This means that
assuming the database connection is providing for atomicity within its
transactional settings, if any individual DML statement inside the flush fails,
the entire operation will be rolled back.
When a failure occurs within a flush, in order to continue using that
same Session, an explicit call to Session.rollback() is
required after a flush fails, even though the underlying transaction will have
been rolled back already (even if the database driver is technically in
driver-level autocommit mode). This is so that the overall nesting pattern of
so-called “subtransactions” is consistently maintained. The FAQ section
“This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar) contains a more detailed description of this
behavior.
See also
“This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar) - further background on why
Session.rollback() must be called when a flush fails.
Get by Primary Key¶
As the Session makes use of an identity map which refers
to current in-memory objects by primary key, the Session.get()
method is provided as a means of locating objects by primary key, first
looking within the current identity map and then querying the database
for non present values. Such as, to locate a User entity with primary key
identity (5, ):
my_user = session.get(User, 5)The Session.get() also includes calling forms for composite primary
key values, which may be passed as tuples or dictionaries, as well as
additional parameters which allow for specific loader and execution options.
See Session.get() for the complete parameter list.
See also
Expiring / Refreshing¶
An important consideration that will often come up when using the
Session is that of dealing with the state that is present on
objects that have been loaded from the database, in terms of keeping them
synchronized with the current state of the transaction. The SQLAlchemy
ORM is based around the concept of an identity map such that when
an object is “loaded” from a SQL query, there will be a unique Python
object instance maintained corresponding to a particular database identity.
This means if we emit two separate queries, each for the same row, and get
a mapped object back, the two queries will have returned the same Python
object:
>>> u1 = session.scalars(select(User).where(User.id == 5)).one()
>>> u2 = session.scalars(select(User).where(User.id == 5)).one()
>>> u1 is u2
TrueFollowing from this, when the ORM gets rows back from a query, it will skip the population of attributes for an object that’s already loaded. The design assumption here is to assume a transaction that’s perfectly isolated, and then to the degree that the transaction isn’t isolated, the application can take steps on an as-needed basis to refresh objects from the database transaction. The FAQ entry at I’m re-loading data with my Session but it isn’t seeing changes that I committed elsewhere discusses this concept in more detail.
When an ORM mapped object is loaded into memory, there are three general ways to refresh its contents with new data from the current transaction:
the expire() method - the
Session.expire()method will erase the contents of selected or all attributes of an object, such that they will be loaded from the database when they are next accessed, e.g. using a lazy loading pattern:session.expire(u1) u1.some_attribute # <-- lazy loads from the transaction
the refresh() method - closely related is the
Session.refresh()method, which does everything theSession.expire()method does but also emits one or more SQL queries immediately to actually refresh the contents of the object:session.refresh(u1) # <-- emits a SQL query u1.some_attribute # <-- is refreshed from the transaction
the populate_existing() method or execution option - This is now an execution option documented at Populate Existing; in legacy form it’s found on the
Queryobject as theQuery.populate_existing()method. This operation in either form indicates that objects being returned from a query should be unconditionally re-populated from their contents in the database:u2 = session.scalars( select(User).where(User.id == 5).execution_options(populate_existing=True) ).one()
Further discussion on the refresh / expire concept can be found at Refreshing / Expiring.
UPDATE and DELETE with arbitrary WHERE clause¶
SQLAlchemy 2.0 includes enhanced capabilities for emitting several varieties of ORM-enabled INSERT, UPDATE and DELETE statements. See the document at ORM-Enabled INSERT, UPDATE, and DELETE statements for documentation.
Auto Begin¶
The Session object features a behavior known as autobegin.
This indicates that the Session will internally consider itself
to be in a “transactional” state as soon as any work is performed with the
Session, either involving modifications to the internal state of
the Session with regards to object state changes, or with
operations that require database connectivity.
When the Session is first constructed, there’s no transactional
state present. The transactional state is begun automatically, when
a method such as Session.add() or Session.execute()
is invoked, or similarly if a Query is executed to return
results (which ultimately uses Session.execute()), or if
an attribute is modified on a persistent object.
The transactional state can be checked by accessing the
Session.in_transaction() method, which returns True or False
indicating if the “autobegin” step has proceeded. While not normally needed,
the Session.get_transaction() method will return the actual
SessionTransaction object that represents this transactional
state.
The transactional state of the Session may also be started
explicitly, by invoking the Session.begin() method. When this
method is called, the Session is placed into the “transactional”
state unconditionally. Session.begin() may be used as a context
manager as described at Framing out a begin / commit / rollback block.
Disabling Autobegin to Prevent Implicit Transactions¶
The “autobegin” behavior may be disabled using the
Session.autobegin parameter set to False. By using this
parameter, a Session will require that the
Session.begin() method is called explicitly. Upon construction, as
well as after any of the Session.rollback(),
Session.commit(), or Session.close() methods are called,
the Session won’t implicitly begin any new transactions and will
raise an error if an attempt to use the Session is made without
first calling Session.begin():
with Session(engine, autobegin=False) as session:
session.begin() # <-- required, else InvalidRequestError raised on next call
session.add(User(name="u1"))
session.commit()
session.begin() # <-- required, else InvalidRequestError raised on next call
u1 = session.scalar(select(User).filter_by(name="u1"))New in version 2.0: Added Session.autobegin, allowing
“autobegin” behavior to be disabled
Committing¶
Session.commit() is used to commit the current
transaction. At its core this indicates that it emits COMMIT on
all current database connections that have a transaction in progress;
from a DBAPI perspective this means the connection.commit()
DBAPI method is invoked on each DBAPI connection.
When there is no transaction in place for the Session, indicating
that no operations were invoked on this Session since the previous
call to Session.commit(), the method will begin and commit an
internal-only “logical” transaction, that does not normally affect the database
unless pending flush changes were detected, but will still invoke event
handlers and object expiration rules.
The Session.commit() operation unconditionally issues
Session.flush() before emitting COMMIT on relevant database
connections. If no pending changes are detected, then no SQL is emitted to the
database. This behavior is not configurable and is not affected by the
Session.autoflush parameter.
Subsequent to that, Session.commit() will then COMMIT the actual
database transaction or transactions, if any, that are in place.
Finally, all objects within the Session are expired as
the transaction is closed out. This is so that when the instances are next
accessed, either through attribute access or by them being present in the
result of a SELECT, they receive the most recent state. This behavior may be
controlled by the Session.expire_on_commit flag, which may be
set to False when this behavior is undesirable.
See also
Rolling Back¶
Session.rollback() rolls back the current transaction, if any.
When there is no transaction in place, the method passes silently.
With a default configured session, the
post-rollback state of the session, subsequent to a transaction having
been begun either via autobegin
or by calling the Session.begin()
method explicitly, is as follows:
All transactions are rolled back and all connections returned to the connection pool, unless the Session was bound directly to a Connection, in which case the connection is still maintained (but still rolled back).
Objects which were initially in the pending state when they were added to the
Sessionwithin the lifespan of the transaction are expunged, corresponding to their INSERT statement being rolled back. The state of their attributes remains unchanged.Objects which were marked as deleted within the lifespan of the transaction are promoted back to the persistent state, corresponding to their DELETE statement being rolled back. Note that if those objects were first pending within the transaction, that operation takes precedence instead.
All objects not expunged are fully expired - this is regardless of the
Session.expire_on_commitsetting.
With that state understood, the Session may
safely continue usage after a rollback occurs.
Changed in version 1.4: The Session object now features deferred “begin” behavior, as
described in autobegin. If no transaction is
begun, methods like Session.commit() and
Session.rollback() have no effect. This behavior would not
have been observed prior to 1.4 as under non-autocommit mode, a
transaction would always be implicitly present.
When a Session.flush() fails, typically for reasons like primary
key, foreign key, or “not nullable” constraint violations, a ROLLBACK is issued
automatically (it’s currently not possible for a flush to continue after a
partial failure). However, the Session goes into a state known as
“inactive” at this point, and the calling application must always call the
Session.rollback() method explicitly so that the
Session can go back into a usable state (it can also be simply
closed and discarded). See the FAQ entry at “This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar) for
further discussion.
See also
Closing¶
The Session.close() method issues a Session.expunge_all() which
removes all ORM-mapped objects from the session, and releases any
transactional/connection resources from the Engine object(s)
to which it is bound. When connections are returned to the connection pool,
transactional state is rolled back as well.
By default, when the Session is closed, it is essentially in the
original state as when it was first constructed, and may be used again.
In this sense, the Session.close() method is more like a “reset”
back to the clean state and not as much like a “database close” method.
In this mode of operation the method Session.reset() is an alias to
Session.close() and behaves in the same way.
The default behavior of Session.close() can be changed by setting the
parameter Session.close_resets_only to False, indicating that
the Session cannot be reused after the method
Session.close() has been called. In this mode of operation the
Session.reset() method will allow multiple “reset” of the session,
behaving like Session.close() when
Session.close_resets_only is set to True.
New in version 2.0.22.
It’s recommended that the scope of a Session be limited by
a call to Session.close() at the end, especially if the
Session.commit() or Session.rollback() methods are not
used. The Session may be used as a context manager to ensure
that Session.close() is called:
with Session(engine) as session:
result = session.execute(select(User))
# closes session automaticallyChanged in version 1.4: The Session object features deferred “begin” behavior, as
described in autobegin. no longer immediately
begins a new transaction after the Session.close() method is
called.
Session Frequently Asked Questions¶
By this point, many users already have questions about sessions.
This section presents a mini-FAQ (note that we have also a real FAQ)
of the most basic issues one is presented with when using a Session.
When do I make a sessionmaker?¶
Just one time, somewhere in your application’s global scope. It should be
looked upon as part of your application’s configuration. If your
application has three .py files in a package, you could, for example,
place the sessionmaker line in your __init__.py file; from
that point on your other modules say “from mypackage import Session”. That
way, everyone else just uses Session(),
and the configuration of that session is controlled by that central point.
If your application starts up, does imports, but does not know what
database it’s going to be connecting to, you can bind the
Session at the “class” level to the
engine later on, using sessionmaker.configure().
In the examples in this section, we will frequently show the
sessionmaker being created right above the line where we actually
invoke Session. But that’s just for
example’s sake! In reality, the sessionmaker would be somewhere
at the module level. The calls to instantiate Session
would then be placed at the point in the application where database
conversations begin.
When do I construct a Session, when do I commit it, and when do I close it?¶
A Session is typically constructed at the beginning of a logical
operation where database access is potentially anticipated.
The Session, whenever it is used to talk to the database,
begins a database transaction as soon as it starts communicating.
This transaction remains in progress until the Session
is rolled back, committed, or closed. The Session will
begin a new transaction if it is used again, subsequent to the previous
transaction ending; from this it follows that the Session
is capable of having a lifespan across many transactions, though only
one at a time. We refer to these two concepts as transaction scope
and session scope.
It’s usually not very hard to determine the best points at which
to begin and end the scope of a Session, though the wide
variety of application architectures possible can introduce
challenging situations.
Some sample scenarios include:
Web applications. In this case, it’s best to make use of the SQLAlchemy integrations provided by the web framework in use. Or otherwise, the basic pattern is create a
Sessionat the start of a web request, call theSession.commit()method at the end of web requests that do POST, PUT, or DELETE, and then close the session at the end of web request. It’s also usually a good idea to setSession.expire_on_committo False so that subsequent access to objects that came from aSessionwithin the view layer do not need to emit new SQL queries to refresh the objects, if the transaction has been committed already.A background daemon which spawns off child forks would want to create a
Sessionlocal to each child process, work with thatSessionthrough the life of the “job” that the fork is handling, then tear it down when the job is completed.For a command-line script, the application would create a single, global
Sessionthat is established when the program begins to do its work, and commits it right as the program is completing its task.For a GUI interface-driven application, the scope of the
Sessionmay best be within the scope of a user-generated event, such as a button push. Or, the scope may correspond to explicit user interaction, such as the user “opening” a series of records, then “saving” them.
As a general rule, the application should manage the lifecycle of the session externally to functions that deal with specific data. This is a fundamental separation of concerns which keeps data-specific operations agnostic of the context in which they access and manipulate that data.
E.g. don’t do this:
### this is the **wrong way to do it** ###
class ThingOne:
def go(self):
session = Session()
try:
session.execute(update(FooBar).values(x=5))
session.commit()
except:
session.rollback()
raise
class ThingTwo:
def go(self):
session = Session()
try:
session.execute(update(Widget).values(q=18))
session.commit()
except:
session.rollback()
raise
def run_my_program():
ThingOne().go()
ThingTwo().go()Keep the lifecycle of the session (and usually the transaction)
separate and external. The example below illustrates how this might look,
and additionally makes use of a Python context manager (i.e. the with:
keyword) in order to manage the scope of the Session and its
transaction automatically:
### this is a **better** (but not the only) way to do it ###
class ThingOne:
def go(self, session):
session.execute(update(FooBar).values(x=5))
class ThingTwo:
def go(self, session):
session.execute(update(Widget).values(q=18))
def run_my_program():
with Session() as session:
with session.begin():
ThingOne().go(session)
ThingTwo().go(session)Changed in version 1.4: The Session may be used as a context
manager without the use of external helper functions.
Is the Session a cache?¶
Yeee…no. It’s somewhat used as a cache, in that it implements the
identity map pattern, and stores objects keyed to their primary key.
However, it doesn’t do any kind of query caching. This means, if you say
session.scalars(select(Foo).filter_by(name='bar')), even if Foo(name='bar')
is right there, in the identity map, the session has no idea about that.
It has to issue SQL to the database, get the rows back, and then when it
sees the primary key in the row, then it can look in the local identity
map and see that the object is already there. It’s only when you say
query.get({some primary key}) that the
Session doesn’t have to issue a query.
Additionally, the Session stores object instances using a weak reference by default. This also defeats the purpose of using the Session as a cache.
The Session is not designed to be a
global object from which everyone consults as a “registry” of objects.
That’s more the job of a second level cache. SQLAlchemy provides
a pattern for implementing second level caching using dogpile.cache,
via the Dogpile Caching example.
How can I get the Session for a certain object?¶
Use the Session.object_session() classmethod
available on Session:
session = Session.object_session(someobject)The newer Runtime Inspection API system can also be used:
from sqlalchemy import inspect
session = inspect(someobject).session