IntegrityError with Django m2m relations

Question

I have a relatively simple Django app, with quite heavy usage that is responsible for quite some concurrency in the db operations.

I have a model Post with a m2m to a Tag model.

A single line in my code, p.add(t) is repeatedly causing mysql exceptions (where p is a Post instance and t is a Tag instance.)

IntegrityError: (1062, "Duplicate entry '329051-1827414' for key 'post_id'")

When this is raised I can manually run this p.add(t) successfully, so it must have to do with some peculiar state that the db/app are in at the time of normal execution. It happens about once every 1000 tag-adding attempts, without any pattern that I can detect (i.e both numbers in the "329051-1827414" pair of the example change)

A CHECK TABLE in mysql on the relevant table shows that they are all seemingly OK.

Any ideas?

Answer 1

Usually you see errors like that when trying to add to an intermediate table if the row being added duplicates the unique-together constraint for the FK's. I'm guessing that in the example you provided "329051" is a Post id and "1827414" is a Tag id.

Normally in Django you can call the add() method repeatedly to add the same instance and Django takes care of everything for you. I'm assuming the model manager maintains some state to help it determine if each add() represents a new or existing row and if the row appears to be new it attempts an insert.

That in itself doesn't explain why you're getting the error. You mention "is responsible for quite some concurrency in the db operations.". Without knowing what that means, I'm guessing that you could be getting a race condition where multiple thread/processes are attempting to add the same new tag around the same time and both are attempting inserts.