Understanding Django Constraint Detection in Custom Fields

The behavior you're experiencing with your custom ChoiceField is related to how Django's migration system detects and processes model changes.

Why Your Constraints Aren't Being Detected

When you add constraints directly to cls._meta.constraints in your contribute_to_class method, Django's migration system doesn't automatically detect these changes unless the constraints attribute has already been initialized in the model's Meta class.

This happens because Django's migration framework compares the current state of your models with their previous state to determine what changes need to be made. When you don't define a Meta class with an empty constraints list, Django doesn't have a baseline to compare against, so it doesn't recognize that constraints have been added programmatically.

Solution and Explanation

Your current workaround (requiring an empty constraints list in the model's Meta) is the correct approach given Django's current implementation. Here's why:

1. When you define constraints = [] in the model's Meta class, Django initializes the attribute and tracks it.
2. When your field's contribute_to_class method adds constraints to this list, Django can detect the difference between the empty list and the list with your added constraint.

Alternative Approaches

If you want to avoid requiring users of your field to add an empty constraints list, you could:

1. Create a model mixin that includes the Meta class with empty constraints
2. Use a custom migration operation
3. Create a system check that warns users if they're using your field without the required Meta setup

Example Implementation with a Mixin

PYTHONCopied
class ChoiceConstraintMixin:
    class Meta:
        constraints = []

class MyModel(ChoiceConstraintMixin, models.Model):
    my_field = ChoiceField(choices=[('a', 'A'), ('b', 'B')])
    # No need to define Meta with empty constraints

Regarding Native Support

Django currently doesn't have built-in support for automatically enforcing choices at the database level. Your custom field is a good approach to fill this gap, especially for cases where data might be modified outside of Django's ORM.

The placeholder naming convention you're using (%(app_label)s_%(class)s_{name}_valid_choices) is the correct approach for handling constraints in reusable fields, as it follows Django's recommended pattern for avoiding name collisions in abstract base classes.