Regarding db_constraint, I think this is a bug

Describe the bug
If the purpose of db_constraint is to avoid generating constraints in the database, then it does not seem to work as intended.

To Reproduce
Create the following data model and run the database migration:

class Feedback(Model):
    id = fields.IntField(primary_key=True)
    user = fields.ForeignKeyField('app.User', related_name='feedbacks', db_constraint=False) 
    title = fields.CharField(default='', max_length=50)

Expected behavior: No foreign key constraint exists in the database.
Actual behavior: Not only does a foreign key constraint get created, it is even more standardized and reliable than when db_constraint=True 😅
My actual requirement is to skip this physical database constraint entirely, while only retaining the relational annotation at the data model layer, with full support for prefetch_related, select_related and values() queries.

Reproduce the migration with the Feedback model shown, then trace how ForeignKeyField and db_constraint=False are handled during database schema generation. Confirm that no physical foreign-key constraint is created while prefetch_related, select_related, and values() continue to support the relationship.