How to bind event on create of object in SqlAlchemy?

Question

I have two tables A and B. I need to create a row in table B (with ForeignKey reference), after I create a new row in table A.

For example:

class A(db.Model):
    __tablename__ = 'a'

    id = Column(BigInteger, primary_key=True)
    ...

class B(db.Model):
    __tablename__ = 'b'

    id = Column(BigInteger, primary_key=True)
    a_id = Column(BigInteger, ForeignKey('a.id'), nullable=False, index=True)
    ...

What is the best solution for this case in SA?

I heard about events, but can't understand how to use them the best way in this case.

Jeff Widman · Accepted Answer · 2015-11-16 07:35:27Z

3

A sample event :

def update_slug(mapper, connection, target):
    target.slug = slugify(target.name)
    if target.name_en:
        target.slug_en = slugify(target.name_en)

event.listen(Post, 'before_insert', update_slug_with_date)
event.listen(Post, 'before_update', update_slug_with_date)

You just define a function. You need to mark it as an event (last two lines), just in your case it will be 'after_insert' and probably 'after update' on the A model. In the function you can do something like this :

b = B()
b.a = target
b.save()

target here is the instance on which event is being raised.

EDIT FROM THE COMMENT TO MARK THE RIGHT ANSWER : According to docs you can't do it in event but in SessionEvents.after_flush . You should check out the docs : http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.MapperEvents.after_insert

edited Nov 16, 2015 at 7:35

Jeff Widman

23.9k13 gold badges79 silver badges93 bronze badges

answered Nov 22, 2013 at 13:53

Paweł Pogorzelski

6516 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Ellochka Cannibal Over a year ago

Pawel, Thx for the help! Could you tell me why even if I do commit inside the event listener I get the TransactionClosed error? Can I use transaction inside and rollback on exception? I can't find in documentation what types of actions can I use inside event listener (for example, session commit, sql expressions, orm actions, new transactions and etc.)

Paweł Pogorzelski Over a year ago

Sorry. My pointer was somewhat wrong (short term memory ;) ) : According to docs you can't do it in event but in SessionEvents.after_flush . You should check out the docs in docs.sqlalchemy.org/en/rel_0_8/orm/…

Collectives™ on Stack Overflow

How to bind event on create of object in SqlAlchemy?

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related