Testing with multiple databases assumes sites app installed on all of them

[Ryan Fugger]

I am trying to run tests on a project with two databases. One of them has all the default django apps installed on them, including sites. The other has some of my own apps. When creating the test databases, I get the following error:

  [...]
  File "/home/ryan/projects/villages/django/db/backends/creation.py", line 256, in create_test_db
    if Site is not None and Site.objects.using(self.connection.alias).count() == 1:
  File "/home/ryan/projects/villages/django/db/models/query.py", line 331, in count
    return self.query.get_count(using=self.db)
  File "/home/ryan/projects/villages/django/db/models/sql/query.py", line 406, in get_count
    number = obj.get_aggregation(using=using)[None]
  File "/home/ryan/projects/villages/django/db/models/sql/query.py", line 372, in get_aggregation
    result = query.get_compiler(using).execute_sql(SINGLE)
  File "/home/ryan/projects/villages/django/db/models/sql/compiler.py", line 754, in execute_sql
    cursor.execute(sql, params)
  File "/home/ryan/projects/villages/django/db/backends/postgresql_psycopg2/base.py", line 43, in execute
    return self.cursor.execute(query, args)
django.db.utils.DatabaseError: relation "django_site" does not exist
LINE 1: SELECT COUNT(*) FROM "django_site"

The problem seems to come from django.db.backends.creation.BaseDatabaseCreation.create_test_db, line 256, which checks if sites is installed, and then proceeds to query the sites table on the current database without checking that the sites table has even been created on the current database.

        Site = get_model('sites', 'Site')
  --->  if Site is not None and Site.objects.using(self.connection.alias).count() == 1:
            Site.objects.using(self.connection.alias).update(id=settings.SITE_ID)

This code was introduced in [16027].

[Aymeric Augustin]

This really looks like #16353.