When adding issue relations, under certain conditions, it is not possible for the model validations to check whether a new relation can be added. This results in an @ActiveRecord::RecordNotUnique@ error to be thrown due to the UNIQUE index on the database.

There are two cases where this can occur:

The first case are race conditions with multiple quick attempts to add the relation which end up on multiple worker processes and are handled concurrently. This is a common (but generally rather unlikely) issue with all unique validations on models.

The second case (which in this case is a logic issue) is triggered if there already are directed relations between issues and a user tries to add the reverse relation. Consider this case:

• There exists a precedes relation between issues 1 and 2.
• The user tries to add a follows relation between issues 2 and 1 ** After validation but before save, the new relation is normalized in @Relation#handle_issue_order@ to be saved as "1 precedes 2". ** A save of this normalized relation triggers the uniqueness constraint on the database and results in an unhandled @ActiveRecord::RecordNotUnique@ error

The root cause of the issue is that the normalization happens after validation. This was moved from @before_validation@ to @@before_save@ in r3191. I guess his was done so that validation errors do not alter the displayed form with the validation errors.

The attached patch (which I extracted from "Planio":https://plan.io/redmine-hosting/) tries to deal with this situation by handling the exception in the controller and to add a generic error to the model. This can handle both cases described above without an unhandled exception by showing a suitable error message to all clients.