DjangoRestFramework ModelSerializer: field-level validation is not working

Field-level validation is called before serializer-level validation.

So model User having username as unique=True, the field-level validation will raise exception because of username being already present. DRF's UniqueValidator does this work of raising exception when a field is not unique.

As per DRF source code,

class UniqueValidator:
    """
    Validator that corresponds to `unique=True` on a model field.

    Should be applied to an individual field on the serializer.
    """
    message = _('This field must be unique.')

Since these validators run before serializer-level validation, your validate_username is never called.