id summary reporter owner description type status component version severity resolution keywords cc stage has_patch needs_docs needs_tests needs_better_patch easy ui_ux 25493 Model instances created with unittest.mock can raise confusing errors Josh Smeaton Marcelo Galigniana "Django can raise one of the following - `ValueError: Failed to insert expression` or; - `ValueError: Failed to insert expression """" on fundraising.Payment.stripe_charge_id. F() expressions can only be used to update, not to insert.` when using unittest.mock to mock the creation of an object. The djangoproject.com tests began throwing errors. Tim fixed these problems in this PR: https://github.com/django/djangoproject.com/pull/524 The PR adds return values for the id of columns that are used to create new instances to avoid these errors. What I think is happening is that the lack of id is somehow causing the mock to be treated as a `Col`. The existence of a `Col` in a `objects.create()` call is what triggers the confusing error message. Since no `F()` expression has been used by the user, tracking down the cause is difficult. We should try and figure out why the mock is being resolved to a Col and stop that from happening or improve the error message if that's not possible." Cleanup/optimization closed Database layer (models, ORM) 1.9 Normal wontfix josh.smeaton@… Accepted 0 0 0 0 0 0