SQLAlchemy 2.0 Documentation
Frequently Asked Questions
- Installation
- Connections / Engines
- MetaData / Schema
- SQL Expressions
- ORM Configuration
- Performance
- Sessions / Queries¶
- I’m re-loading data with my Session but it isn’t seeing changes that I committed elsewhere
- “This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar)
- How do I make a Query that always adds a certain filter to every query?
- My Query does not return the same number of objects as query.count() tells me - why?
- I’ve created a mapping against an Outer Join, and while the query returns rows, no objects are returned. Why not?
- I’m using
joinedload()
orlazy=False
to create a JOIN/OUTER JOIN and SQLAlchemy is not constructing the correct query when I try to add a WHERE, ORDER BY, LIMIT, etc. (which relies upon the (OUTER) JOIN) - Query has no
__len__()
, why not? - How Do I use Textual SQL with ORM Queries?
- I’m calling
Session.delete(myobject)
and it isn’t removed from the parent collection! - why isn’t my
__init__()
called when I load objects? - how do I use ON DELETE CASCADE with SA’s ORM?
- I set the “foo_id” attribute on my instance to “7”, but the “foo” attribute is still
None
- shouldn’t it have loaded Foo with id #7? - How do I walk all objects that are related to a given object?
- Is there a way to automagically have only unique keywords (or other kinds of objects) without doing a query for the keyword and getting a reference to the row containing that keyword?
- Why does post_update emit UPDATE in addition to the first UPDATE?
- Third Party Integration Issues
Project Versions
- Previous: Performance
- Next: Third Party Integration Issues
- Up: Home
- On this page:
- Sessions / Queries
- I’m re-loading data with my Session but it isn’t seeing changes that I committed elsewhere
- “This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar)
- How do I make a Query that always adds a certain filter to every query?
- My Query does not return the same number of objects as query.count() tells me - why?
- I’ve created a mapping against an Outer Join, and while the query returns rows, no objects are returned. Why not?
- I’m using
joinedload()
orlazy=False
to create a JOIN/OUTER JOIN and SQLAlchemy is not constructing the correct query when I try to add a WHERE, ORDER BY, LIMIT, etc. (which relies upon the (OUTER) JOIN) - Query has no
__len__()
, why not? - How Do I use Textual SQL with ORM Queries?
- I’m calling
Session.delete(myobject)
and it isn’t removed from the parent collection! - why isn’t my
__init__()
called when I load objects? - how do I use ON DELETE CASCADE with SA’s ORM?
- I set the “foo_id” attribute on my instance to “7”, but the “foo” attribute is still
None
- shouldn’t it have loaded Foo with id #7? - How do I walk all objects that are related to a given object?
- Is there a way to automagically have only unique keywords (or other kinds of objects) without doing a query for the keyword and getting a reference to the row containing that keyword?
- Why does post_update emit UPDATE in addition to the first UPDATE?
Sessions / Queries¶
I’m re-loading data with my Session but it isn’t seeing changes that I committed elsewhere¶
The main issue regarding this behavior is that the session acts as though the transaction is in the serializable isolation state, even if it’s not (and it usually is not). In practical terms, this means that the session does not alter any data that it’s already read within the scope of a transaction.
If the term “isolation level” is unfamiliar, then you first need to read this link:
In short, serializable isolation level generally means that once you SELECT a series of rows in a transaction, you will get the identical data back each time you re-emit that SELECT. If you are in the next-lower isolation level, “repeatable read”, you’ll see newly added rows (and no longer see deleted rows), but for rows that you’ve already loaded, you won’t see any change. Only if you are in a lower isolation level, e.g. “read committed”, does it become possible to see a row of data change its value.
For information on controlling the isolation level when using the SQLAlchemy ORM, see Setting Transaction Isolation Levels / DBAPI AUTOCOMMIT.
To simplify things dramatically, the Session
itself works in
terms of a completely isolated transaction, and doesn’t overwrite any mapped attributes
it’s already read unless you tell it to. The use case of trying to re-read
data you’ve already loaded in an ongoing transaction is an uncommon use
case that in many cases has no effect, so this is considered to be the
exception, not the norm; to work within this exception, several methods
are provided to allow specific data to be reloaded within the context
of an ongoing transaction.
To understand what we mean by “the transaction” when we talk about the
Session
, your Session
is intended to only work within
a transaction. An overview of this is at Managing Transactions.
Once we’ve figured out what our isolation level is, and we think that
our isolation level is set at a low enough level so that if we re-SELECT a row,
we should see new data in our Session
, how do we see it?
Three ways, from most common to least:
We simply end our transaction and start a new one on next access with our
Session
by callingSession.commit()
(note that if theSession
is in the lesser-used “autocommit” mode, there would be a call toSession.begin()
as well). The vast majority of applications and use cases do not have any issues with not being able to “see” data in other transactions because they stick to this pattern, which is at the core of the best practice of short lived transactions. See When do I construct a Session, when do I commit it, and when do I close it? for some thoughts on this.We tell our
Session
to re-read rows that it has already read, either when we next query for them usingSession.expire_all()
orSession.expire()
, or immediately on an object usingrefresh
. See Refreshing / Expiring for detail on this.We can run whole queries while setting them to definitely overwrite already-loaded objects as they read rows by using “populate existing”. This is an execution option described at Populate Existing.
But remember, the ORM cannot see changes in rows if our isolation level is repeatable read or higher, unless we start a new transaction.
“This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar)¶
This is an error that occurs when a Session.flush()
raises an exception, rolls back
the transaction, but further commands upon the Session
are called without an
explicit call to Session.rollback()
or Session.close()
.
It usually corresponds to an application that catches an exception
upon Session.flush()
or Session.commit()
and
does not properly handle the exception. For example:
from sqlalchemy import create_engine, Column, Integer
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base(create_engine("sqlite://"))
class Foo(Base):
__tablename__ = "foo"
id = Column(Integer, primary_key=True)
Base.metadata.create_all()
session = sessionmaker()()
# constraint violation
session.add_all([Foo(id=1), Foo(id=1)])
try:
session.commit()
except:
# ignore error
pass
# continue using session without rolling back
session.commit()
The usage of the Session
should fit within a structure similar to this:
try:
# <use session>
session.commit()
except:
session.rollback()
raise
finally:
session.close() # optional, depends on use case
Many things can cause a failure within the try/except besides flushes. Applications should ensure some system of “framing” is applied to ORM-oriented processes so that connection and transaction resources have a definitive boundary, and so that transactions can be explicitly rolled back if any failure conditions occur.
This does not mean there should be try/except blocks throughout an application, which would not be a scalable architecture. Instead, a typical approach is that when ORM-oriented methods and functions are first called, the process that’s calling the functions from the very top would be within a block that commits transactions at the successful completion of a series of operations, as well as rolls transactions back if operations fail for any reason, including failed flushes. There are also approaches using function decorators or context managers to achieve similar results. The kind of approach taken depends very much on the kind of application being written.
For a detailed discussion on how to organize usage of the Session
,
please see When do I construct a Session, when do I commit it, and when do I close it?.
But why does flush() insist on issuing a ROLLBACK?¶
It would be great if Session.flush()
could partially complete and then
not roll back, however this is beyond its current capabilities since its
internal bookkeeping would have to be modified such that it can be halted at
any time and be exactly consistent with what’s been flushed to the database.
While this is theoretically possible, the usefulness of the enhancement is
greatly decreased by the fact that many database operations require a ROLLBACK
in any case. Postgres in particular has operations which, once failed, the
transaction is not allowed to continue:
test=> create table foo(id integer primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "foo_pkey" for table "foo"
CREATE TABLE
test=> begin;
BEGIN
test=> insert into foo values(1);
INSERT 0 1
test=> commit;
COMMIT
test=> begin;
BEGIN
test=> insert into foo values(1);
ERROR: duplicate key value violates unique constraint "foo_pkey"
test=> insert into foo values(2);
ERROR: current transaction is aborted, commands ignored until end of transaction block
What SQLAlchemy offers that solves both issues is support of SAVEPOINT, via
Session.begin_nested()
. Using Session.begin_nested()
, you can frame an operation that may
potentially fail within a transaction, and then “roll back” to the point
before its failure while maintaining the enclosing transaction.
But why isn’t the one automatic call to ROLLBACK enough? Why must I ROLLBACK again?¶
The rollback that’s caused by the flush() is not the end of the complete transaction
block; while it ends the database transaction in play, from the Session
point of view there is still a transaction that is now in an inactive state.
Given a block such as:
sess = Session() # begins a logical transaction
try:
sess.flush()
sess.commit()
except:
sess.rollback()
Above, when a Session
is first created, assuming “autocommit mode”
isn’t used, a logical transaction is established within the Session
.
This transaction is “logical” in that it does not actually use any database
resources until a SQL statement is invoked, at which point a connection-level
and DBAPI-level transaction is started. However, whether or not
database-level transactions are part of its state, the logical transaction will
stay in place until it is ended using Session.commit()
,
Session.rollback()
, or Session.close()
.
When the flush()
above fails, the code is still within the transaction
framed by the try/commit/except/rollback block. If flush()
were to fully
roll back the logical transaction, it would mean that when we then reach the
except:
block the Session
would be in a clean state, ready to
emit new SQL on an all new transaction, and the call to
Session.rollback()
would be out of sequence. In particular, the
Session
would have begun a new transaction by this point, which the
Session.rollback()
would be acting upon erroneously. Rather than
allowing SQL operations to proceed on a new transaction in this place where
normal usage dictates a rollback is about to take place, the Session
instead refuses to continue until the explicit rollback actually occurs.
In other words, it is expected that the calling code will always call
Session.commit()
, Session.rollback()
, or Session.close()
to correspond to the current transaction block. flush()
keeps the
Session
within this transaction block so that the behavior of the
above code is predictable and consistent.
How do I make a Query that always adds a certain filter to every query?¶
See the recipe at FilteredQuery.
My Query does not return the same number of objects as query.count() tells me - why?¶
The Query
object, when asked to return a list of ORM-mapped objects,
will deduplicate the objects based on primary key. That is, if we
for example use the User
mapping described at Using ORM Declarative Forms to Define Table Metadata,
and we had a SQL query like the following:
q = session.query(User).outerjoin(User.addresses).filter(User.name == "jack")
Above, the sample data used in the tutorial has two rows in the addresses
table for the users
row with the name 'jack'
, primary key value 5.
If we ask the above query for a Query.count()
, we will get the answer
2:
>>> q.count()
2
However, if we run Query.all()
or iterate over the query, we get back
one element:
>>> q.all()
[User(id=5, name='jack', ...)]
This is because when the Query
object returns full entities, they
are deduplicated. This does not occur if we instead request individual
columns back:
>>> session.query(User.id, User.name).outerjoin(User.addresses).filter(
... User.name == "jack"
... ).all()
[(5, 'jack'), (5, 'jack')]
There are two main reasons the Query
will deduplicate:
To allow joined eager loading to work correctly - Joined Eager Loading works by querying rows using joins against related tables, where it then routes rows from those joins into collections upon the lead objects. In order to do this, it has to fetch rows where the lead object primary key is repeated for each sub-entry. This pattern can then continue into further sub-collections such that a multiple of rows may be processed for a single lead object, such as
User(id=5)
. The dedpulication allows us to receive objects in the way they were queried, e.g. all theUser()
objects whose name is'jack'
which for us is one object, with theUser.addresses
collection eagerly loaded as was indicated either bylazy='joined'
on therelationship()
or via thejoinedload()
option. For consistency, the deduplication is still applied whether or not the joinedload is established, as the key philosophy behind eager loading is that these options never affect the result.To eliminate confusion regarding the identity map - this is admittedly the less critical reason. As the
Session
makes use of an identity map, even though our SQL result set has two rows with primary key 5, there is only oneUser(id=5)
object inside theSession
which must be maintained uniquely on its identity, that is, its primary key / class combination. It doesn’t actually make much sense, if one is querying forUser()
objects, to get the same object multiple times in the list. An ordered set would potentially be a better representation of whatQuery
seeks to return when it returns full objects.
The issue of Query
deduplication remains problematic, mostly for the
single reason that the Query.count()
method is inconsistent, and the
current status is that joined eager loading has in recent releases been
superseded first by the “subquery eager loading” strategy and more recently the
“select IN eager loading” strategy, both of which are generally more
appropriate for collection eager loading. As this evolution continues,
SQLAlchemy may alter this behavior on Query
, which may also involve
new APIs in order to more directly control this behavior, and may also alter
the behavior of joined eager loading in order to create a more consistent usage
pattern.
I’ve created a mapping against an Outer Join, and while the query returns rows, no objects are returned. Why not?¶
Rows returned by an outer join may contain NULL for part of the primary key,
as the primary key is the composite of both tables. The Query
object ignores incoming rows
that don’t have an acceptable primary key. Based on the setting of the allow_partial_pks
flag on Mapper
, a primary key is accepted if the value has at least one non-NULL
value, or alternatively if the value has no NULL values. See allow_partial_pks
at Mapper
.
I’m using joinedload()
or lazy=False
to create a JOIN/OUTER JOIN and SQLAlchemy is not constructing the correct query when I try to add a WHERE, ORDER BY, LIMIT, etc. (which relies upon the (OUTER) JOIN)¶
The joins generated by joined eager loading are only used to fully load related collections, and are designed to have no impact on the primary results of the query. Since they are anonymously aliased, they cannot be referenced directly.
For detail on this behavior, see The Zen of Joined Eager Loading.
Query has no __len__()
, why not?¶
The Python __len__()
magic method applied to an object allows the len()
builtin to be used to determine the length of the collection. It’s intuitive
that a SQL query object would link __len__()
to the Query.count()
method, which emits a SELECT COUNT. The reason this is not possible is
because evaluating the query as a list would incur two SQL calls instead of
one:
class Iterates:
def __len__(self):
print("LEN!")
return 5
def __iter__(self):
print("ITER!")
return iter([1, 2, 3, 4, 5])
list(Iterates())
output:
ITER!
LEN!
How Do I use Textual SQL with ORM Queries?¶
See:
Getting ORM Results from Textual Statements - Ad-hoc textual blocks with
Query
Using SQL Expressions with Sessions - Using
Session
with textual SQL directly.
I’m calling Session.delete(myobject)
and it isn’t removed from the parent collection!¶
See Notes on Delete - Deleting Objects Referenced from Collections and Scalar Relationships for a description of this behavior.
why isn’t my __init__()
called when I load objects?¶
See Maintaining Non-Mapped State Across Loads for a description of this behavior.
how do I use ON DELETE CASCADE with SA’s ORM?¶
SQLAlchemy will always issue UPDATE or DELETE statements for dependent
rows which are currently loaded in the Session
. For rows which
are not loaded, it will by default issue SELECT statements to load
those rows and update/delete those as well; in other words it assumes
there is no ON DELETE CASCADE configured.
To configure SQLAlchemy to cooperate with ON DELETE CASCADE, see
Using foreign key ON DELETE cascade with ORM relationships.
I set the “foo_id” attribute on my instance to “7”, but the “foo” attribute is still None
- shouldn’t it have loaded Foo with id #7?¶
The ORM is not constructed in such a way as to support
immediate population of relationships driven from foreign
key attribute changes - instead, it is designed to work the
other way around - foreign key attributes are handled by the
ORM behind the scenes, the end user sets up object
relationships naturally. Therefore, the recommended way to
set o.foo
is to do just that - set it!:
foo = session.get(Foo, 7)
o.foo = foo
Session.commit()
Manipulation of foreign key attributes is of course entirely legal. However,
setting a foreign-key attribute to a new value currently does not trigger
an “expire” event of the relationship()
in which it’s involved. This means
that for the following sequence:
o = session.scalars(select(SomeClass).limit(1)).first()
# assume the existing o.foo_id value is None;
# accessing o.foo will reconcile this as ``None``, but will effectively
# "load" the value of None
assert o.foo is None
# now set foo_id to something. o.foo will not be immediately affected
o.foo_id = 7
o.foo
is loaded with its effective database value of None
when it
is first accessed. Setting
o.foo_id = 7
will have the value of “7” as a pending change, but no flush
has occurred - so o.foo
is still None
:
# attribute is already "loaded" as None, has not been
# reconciled with o.foo_id = 7 yet
assert o.foo is None
For o.foo
to load based on the foreign key mutation is usually achieved
naturally after the commit, which both flushes the new foreign key value
and expires all state:
session.commit() # expires all attributes
foo_7 = session.get(Foo, 7)
# o.foo will lazyload again, this time getting the new object
assert o.foo is foo_7
A more minimal operation is to expire the attribute individually - this can
be performed for any persistent object using Session.expire()
:
o = session.scalars(select(SomeClass).limit(1)).first()
o.foo_id = 7
Session.expire(o, ["foo"]) # object must be persistent for this
foo_7 = session.get(Foo, 7)
assert o.foo is foo_7 # o.foo lazyloads on access
Note that if the object is not persistent but present in the Session
,
it’s known as pending. This means the row for the object has not been
INSERTed into the database yet. For such an object, setting foo_id
does not
have meaning until the row is inserted; otherwise there is no row yet:
new_obj = SomeClass()
new_obj.foo_id = 7
Session.add(new_obj)
# returns None but this is not a "lazyload", as the object is not
# persistent in the DB yet, and the None value is not part of the
# object's state
assert new_obj.foo is None
Session.flush() # emits INSERT
assert new_obj.foo is foo_7 # now it loads
The recipe ExpireRelationshipOnFKChange features an example using SQLAlchemy events in order to coordinate the setting of foreign key attributes with many-to-one relationships.
How do I walk all objects that are related to a given object?¶
An object that has other objects related to it will correspond to the
relationship()
constructs set up between mappers. This code fragment will
iterate all the objects, correcting for cycles as well:
from sqlalchemy import inspect
def walk(obj):
deque = [obj]
seen = set()
while deque:
obj = deque.pop(0)
if obj in seen:
continue
else:
seen.add(obj)
yield obj
insp = inspect(obj)
for relationship in insp.mapper.relationships:
related = getattr(obj, relationship.key)
if relationship.uselist:
deque.extend(related)
elif related is not None:
deque.append(related)
The function can be demonstrated as follows:
Base = declarative_base()
class A(Base):
__tablename__ = "a"
id = Column(Integer, primary_key=True)
bs = relationship("B", backref="a")
class B(Base):
__tablename__ = "b"
id = Column(Integer, primary_key=True)
a_id = Column(ForeignKey("a.id"))
c_id = Column(ForeignKey("c.id"))
c = relationship("C", backref="bs")
class C(Base):
__tablename__ = "c"
id = Column(Integer, primary_key=True)
a1 = A(bs=[B(), B(c=C())])
for obj in walk(a1):
print(obj)
Output:
<__main__.A object at 0x10303b190>
<__main__.B object at 0x103025210>
<__main__.B object at 0x10303b0d0>
<__main__.C object at 0x103025490>
Is there a way to automagically have only unique keywords (or other kinds of objects) without doing a query for the keyword and getting a reference to the row containing that keyword?¶
When people read the many-to-many example in the docs, they get hit with the
fact that if you create the same Keyword
twice, it gets put in the DB twice.
Which is somewhat inconvenient.
This UniqueObject recipe was created to address this issue.
Why does post_update emit UPDATE in addition to the first UPDATE?¶
The post_update feature, documented at Rows that point to themselves / Mutually Dependent Rows, involves that an UPDATE statement is emitted in response to changes to a particular relationship-bound foreign key, in addition to the INSERT/UPDATE/DELETE that would normally be emitted for the target row. While the primary purpose of this UPDATE statement is that it pairs up with an INSERT or DELETE of that row, so that it can post-set or pre-unset a foreign key reference in order to break a cycle with a mutually dependent foreign key, it currently is also bundled as a second UPDATE that emits when the target row itself is subject to an UPDATE. In this case, the UPDATE emitted by post_update is usually unnecessary and will often appear wasteful.
However, some research into trying to remove this “UPDATE / UPDATE” behavior reveals that major changes to the unit of work process would need to occur not just throughout the post_update implementation, but also in areas that aren’t related to post_update for this to work, in that the order of operations would need to be reversed on the non-post_update side in some cases, which in turn can impact other cases, such as correctly handling an UPDATE of a referenced primary key value (see #1063 for a proof of concept).
The answer is that “post_update” is used to break a cycle between two mutually dependent foreign keys, and to have this cycle breaking be limited to just INSERT/DELETE of the target table implies that the ordering of UPDATE statements elsewhere would need to be liberalized, leading to breakage in other edge cases.
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