Database Routing in Django Tests

When working with multiple databases in Django tests, you need to ensure your database router correctly handles migrations during test setup. The issue you're experiencing is common when the router's allow_migrate method isn't properly configured to direct models to their respective databases.

Fixing Your Router Implementation

Your router's allow_migrate method should be the key decision-maker for determining which models can be migrated to which database. Here's how to properly implement it:

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class CustomRouter:
    """
    Router to control database operations for different models
    """
    
    # Define which app_labels or models go to which database
    db1_models = {"app1", "app2"}  # Models that should go to first database
    db2_models = {"app3", "app4"}  # Models that should go to second database
    
    def db_for_read(self, model, **hints):
        app_label = model._meta.app_label
        if app_label in self.db1_models:
            return "db1"
        if app_label in self.db2_models:
            return "db2"
        return None
    
    def db_for_write(self, model, **hints):
        app_label = model._meta.app_label
        if app_label in self.db1_models:
            return "db1"
        if app_label in self.db2_models:
            return "db2"
        return None
    
    def allow_relation(self, obj1, obj2, **hints):
        # Allow relations within the same database
        db1_set = {"db1"}
        db2_set = {"db2"}
        
        if obj1._state.db in db1_set and obj2._state.db in db1_set:
            return True
        if obj1._state.db in db2_set and obj2._state.db in db2_set:
            return True
        return None
    
    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Critical method for test setup - determines which models can be migrated to which database
        """
        # Explicitly control which models go to which database
        if db == "db1" and app_label in self.db1_models:
            return True
        if db == "db2" and app_label in self.db2_models:
            return True
            
        # Prevent models from being created in the wrong database
        if db == "db1" and app_label in self.db2_models:
            return False
        if db == "db2" and app_label in self.db1_models:
            return False
            
        # For any other case, you might want to return None or False depending on your needs
        return None

Configuration in Settings

Make sure your router is properly configured in your settings:

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DATABASE_ROUTERS = ["path.to.CustomRouter"]

Test Configuration

For pytest-django specifically, you might need to ensure that your test settings are correctly loading your router. You can create a specific test settings file or use pytest fixtures to configure the database properly.

In your pytest.ini or conftest.py, you can specify:

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# In conftest.py
@pytest.fixture(scope="session")
def django_db_setup(django_db_setup, django_db_blocker):
    # This ensures your router is used during test setup
    with django_db_blocker.unblock():
        # You can perform additional setup here if needed
        pass

Key Points to Remember

1. The allow_migrate method is the critical decision point during test setup
2. It should explicitly control which models can be migrated to which database
3. Return True to allow migration, False to prevent it, and None to defer to other routers
4. The order of routers in DATABASE_ROUTERS matters - they're processed in the order listed

By implementing a strict allow_migrate method that explicitly controls which models go to which database, you should be able to ensure that your test setup creates tables in the correct databases.