Sqlalchemy: secondary relationship update

Question

I have two tables, say A and B. Both have a primary key id. They have a many-to-many relationship, SEC.

SEC = Table(\'sec\', Base.metadata,
    Column(\'a_id\',          


        

Answer 1

The error you mention is indeed from inserting a conflicting value to the sec table. To be sure that it is from the operation you think it is, not some previous change, turn on SQL logging and check what values is it trying to insert before erroring out.

When overwriting a many-to-many collection value, SQLAlchemy compares the new contents of the collection with the state in the database and correspondingly issues delete and insert statements. Unless you are poking around in SQLAlchemy internals, there should be two ways to encounter this error.

First is concurrent modification: Process 1 fetches the value a.rels and notices that it is empty, meanwhile Process 2 also fetches a.rels, sets it to [b1, b2] and commits flushing the (a,b1),(a,b2) tuples, Process 1 sets a.rels to [b1, b3] noticing that the previous contents was empty and when it tries to flush the sec tuple (a,b1) it gets a duplicate key error. The correct action in such cases is usually to retry the transaction from the top. You can use serializable transaction isolation to instead get a serialization error in this case that is distinct from a business logic error causing a duplicate key error.

The second case happens when you have managed to convince SQLAlchemy that you don't need to know the database state by setting the loading strategy of the rels attribute to noload. This can be done when defining the relationship by adding the lazy='noload' parameter, or when querying, calling .options(noload(A.rels)) on the query. SQLAlchemy will assume that sec table has no matching rows for objects loaded with this strategy in effect.