Clashing table name checks don't account for database routers

[Elizaveta Kishchukova]

This code results in errors during integrity checks when there are apps using models with same m2m table name.

in django/db/models/fields/related.py, lines 1441-1453:

    def _check_table_uniqueness(self, **kwargs):
        if isinstance(self.remote_field.through, six.string_types) or not self.remote_field.through._meta.managed:
            return []
        registered_tables = {
            model._meta.db_table: model
            for model in self.opts.apps.get_models(include_auto_created=True)
            if model != self.remote_field.through and model._meta.managed
        }    
        m2m_db_table = self.m2m_db_table()
        model = registered_tables.get(m2m_db_table)
        # The second condition allows multiple m2m relations on a model if
        # some point to a through model that proxies another through model.
        if model and model._meta.concrete_model != self.remote_field.through._meta.concrete_model:

Example:

Set up:
app1.SomeModel with table name "some_table_name" (using database 1)
app2.SomeModel with table name "some_table_name" (using database 2)
app1.Y with field xs = models.ManyToManyField('X', through='SomeModel')
app2.Y with field xs = models.ManyToManyField('X', through='SomeModel')

ERRORS: 
app1.Y.xs: (fields.E340) The field's intermediary table 'some_table_name' clashes with the table name of 'app2.SomeModel'.
app2.Y.xs: (fields.E340) The field's intermediary table 'some_table_name' clashes with the table name of 'app1.SomeModel'.

Notice how it compares models of different apps. This dictionary ought to at least use something like 'database.table_name' or 'app_name.table_name' for key.

[Tim Graham]

If I understand correctly, the only thing preventing the tables from clashing in your case is the database router. I'm not sure if it's worth trying to have this check consult database routers. This doesn't seem like a common setup, so the best solution might be for you to add that check to SILENCED_SYSTEM_CHECKS.

[Tim Graham]

Feel free to reopen and offer a patch for evaluation if you think the implementation is straightforward. I guess any solution can't be perfect since we won't have access to the instance hint at check time.