Fixed #30396 -- Added system checks for uniqueness of indexes and constraints names.

[cansarigol]

Ticket: #30396

[charettes]

Any thoughts about adjusting the message to make it clear the check is performed on all models and include them in the message like we did for E028.

e.g.

• `'foo' index name is not unique amongst models: '
• 'foo' index name is not unique for model <model> (when len(model list) == 1).

[cansarigol]

thanks @charettes, I applied your comments.

[felixxm]

@cansarigol Thanks for this patch. I left some comments.

[felixxm]

I would use the same mechanism as for the E020 and models' labels instead of __name__'s, i.e.

indexes = defaultdict(list)
constraints = defaultdict(list)
...
for model_index in model._meta.indexes:
    indexes[model_index.name].append(model._meta.label)
for model_constraint in model._meta.constraints:
    constraints[model_constraint.name].append(model._meta.label)

[felixxm]

Maybe:

* **models.E029**: index name ``<index>`` is not unique for model ``<model>``.
  ``<model list>``.
* **models.E030**: index name ``<index>`` is not unique amongst models:
  ``<model list>``.
* **models.E031**: constraint name ``<constraint>`` is not unique for model
  ``<model>``.
* **models.E032**: constraint name ``<constraint>`` is not unique amongst
  models: ``<model list>``.

[cansarigol]

thanks, @felixxm, I changed the pr as your comments. Isn't Error.obj necessary?

[felixxm]

@cansarigol obj is not necessary and in this case it doesn't have much value.

[cansarigol]

@cansarigol obj is not necessary and in this case it doesn't have much value.

you're right. besides that, there are multiple objects in plural case.

[felixxm]

I rebased from master, split tests, and added tests for collisions across different apps and from abstract models.

[cansarigol]

the code is nicer and more inclusive in this way. thank you for your help @felixxm