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:

Isolation Level

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.

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:

1. We simply end our transaction and start a new one on next access with our Session by calling Session.commit() (note that if the Session is in the lesser-used “autocommit” mode, there would be a call to Session.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.

2. We tell our Session to re-read rows that it has already read, either when we next query for them using Session.expire_all() or Session.expire(), or immediately on an object using refresh. See Refreshing / Expiring for detail on this.

3. We can run whole queries while setting them to definitely overwrite already-loaded objects as they read rows by using Query.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. You should always have some kind of “framing” of your session operations so that connection and transaction resources have a definitive boundary, otherwise your application doesn’t really have its usage of resources under control. This is not to say that you need to put try/except blocks all throughout your application - on the contrary, this would be a terrible idea. You should architect your application such that there is one (or few) point(s) of “framing” around session operations.

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?

This is again a matter of the Session providing a consistent interface and refusing to guess about what context its being used. For example, the Session supports “framing” above within multiple levels. Such as, suppose you had a decorator @with_session(), which did this:

def with_session(fn):
   def go(*args, **kw):
       session.begin(subtransactions=True)
       try:
           ret = fn(*args, **kw)
           session.commit()
           return ret
       except:
           session.rollback()
           raise
   return go

The above decorator begins a transaction if one does not exist already, and then commits it, if it were the creator. The “subtransactions” flag means that if Session.begin() were already called by an enclosing function, nothing happens except a counter is incremented - this counter is decremented when Session.commit() is called and only when it goes back to zero does the actual COMMIT happen. It allows this usage pattern:

@with_session
def one():
   # do stuff
   two()


@with_session
def two():
   # etc.

one()

two()

one() can call two(), or two() can be called by itself, and the @with_session decorator ensures the appropriate “framing” - the transaction boundaries stay on the outermost call level. As you can see, if two() calls flush() which throws an exception and then issues a rollback(), there will always be a second rollback() performed by the decorator, and possibly a third corresponding to two levels of decorator. If the flush() pushed the rollback() all the way out to the top of the stack, and then we said that all remaining rollback() calls are moot, there is some silent behavior going on there. A poorly written enclosing method might suppress the exception, and then call commit() assuming nothing is wrong, and then you have a silent failure condition. The main reason people get this error in fact is because they didn’t write clean “framing” code and they would have had other problems down the road.

If you think the above use case is a little exotic, the same kind of thing comes into play if you want to SAVEPOINT- you might call begin_nested() several times, and the commit()/rollback() calls each resolve the most recent begin_nested(). The meaning of rollback() or commit() is dependent upon which enclosing block it is called, and you might have any sequence of rollback()/commit() in any order, and its the level of nesting that determines their behavior.

In both of the above cases, if flush() broke the nesting of transaction blocks, the behavior is, depending on scenario, anywhere from “magic” to silent failure to blatant interruption of code flow.

flush() makes its own “subtransaction”, so that a transaction is started up regardless of the external transactional state, and when complete it calls commit(), or rollback() upon failure - but that rollback() corresponds to its own subtransaction - it doesn’t want to guess how you’d like to handle the external “framing” of the transaction, which could be nested many levels with any combination of subtransactions and real SAVEPOINTs. The job of starting/ending the “frame” is kept consistently with the code external to the flush(), and we made a decision that this was the most consistent approach.

How do I make a Query that always adds a certain filter to every query?

See the recipe at PreFilteredQuery.

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(object):
    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:

• Using Textual SQL - 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 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 Constructors and Object Initialization 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 Passive Deletes.

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.query(Foo).get(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.query(SomeClass).first()
assert o.foo is None  # accessing an un-set attribute sets it to None
o.foo_id = 7

o.foo is initialized to None when we first accessed it. Setting o.foo_id = 7 will have the value of “7” as pending, but no flush has occurred - so o.foo is still None:

# attribute is already set to 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.query(Foo).get(7)

assert o.foo is foo_7  # o.foo lazyloads on access

A more minimal operation is to expire the attribute individually - this can be performed for any persistent object using Session.expire():

o = Session.query(SomeClass).first()
o.foo_id = 7
Session.expire(o, ['foo'])  # object must be persistent for this

foo_7 = Session.query(Foo).get(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)

# accessing an un-set attribute sets it to None
assert new_obj.foo is None

Session.flush()  # emits INSERT

# expire this because we already set .foo to None
Session.expire(o, ['foo'])

assert new_obj.foo is foo_7  # now it loads

Attribute loading for non-persistent objects

One variant on the “pending” behavior above is if we use the flag load_on_pending on relationship(). When this flag is set, the lazy loader will emit for new_obj.foo before the INSERT proceeds; another variant of this is to use the Session.enable_relationship_loading() method, which can “attach” an object to a Session in such a way that many-to-one relationships load as according to foreign key attributes regardless of the object being in any particular state. Both techniques are not recommended for general use; they were added to suit specific programming scenarios encountered by users which involve the repurposing of the ORM’s usual object states.

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.