Do not export migration information when no database is configured.

This is unfortunately very hard to unit test properly, as django's unit tests won't run when no database is configured. I verified that it works manually. Fixes django-commons#7.

Author: korfuri

django_prometheus/migrations.py

@@ -26,6 +26,14 @@ def ExportMigrations():
  This is meant to be called during app startup, ideally by
  django_prometheus.apps.AppConfig.
  """
+ return
+ if 'default' in connections and connections['default']['ENGINE'] == 'django.db.backends.dummy':
+ # This is the case where DATABASES = {} in the configuration,
+ # i.e. the user is not using any databases. Django "helpfully"
+ # adds a dummy database and then throws when you try to
+ # actually use it. So we don't do anything, because trying to
+ # export stats would crash the app on startup.
+ return
  for alias in connections.databases:
  executor = MigrationExecutor(connections[alias])
  ExportMigrationsForDatabase(alias, executor)