fix MongoAutoField ModelForm bugs

.github/workflows/test-python.yml

@@ -97,6 +97,7 @@ jobs:
           m2m_through_regress
           m2o_recursive
           model_fields
+          model_forms
           one_to_one
           ordering
           or_lookups

django_mongodb/features.py

@@ -113,6 +113,7 @@ def django_test_expected_failures(self):
         "QuerySet.prefetch_related() is not supported on MongoDB.": {
             "m2m_through_regress.test_multitable.MultiTableTests.test_m2m_prefetch_proxied",
             "m2m_through_regress.test_multitable.MultiTableTests.test_m2m_prefetch_reverse_proxied",
+            "model_forms.tests.OtherModelFormTests.test_prefetch_related_queryset",
         },
         "QuerySet.update() with expression not supported.": {
             "annotations.tests.AliasTests.test_update_with_alias",
@@ -235,6 +236,11 @@ def django_test_expected_failures(self):
             "lookup.tests.LookupTests.test_exact_exists",
             "lookup.tests.LookupTests.test_nested_outerref_lhs",
             "lookup.tests.LookupQueryingTests.test_filter_exists_lhs",
+            "model_forms.tests.LimitChoicesToTests.test_fields_for_model_applies_limit_choices_to",
+            "model_forms.tests.LimitChoicesToTests.test_limit_choices_to_callable_for_fk_rel",
+            "model_forms.tests.LimitChoicesToTests.test_limit_choices_to_callable_for_m2m_rel",
+            "model_forms.tests.LimitChoicesToTests.test_limit_choices_to_m2m_through",
+            "model_forms.tests.LimitChoicesToTests.test_limit_choices_to_no_duplicates",
             "queries.tests.ExcludeTest17600.test_exclude_plain",
             "queries.tests.ExcludeTest17600.test_exclude_with_q_is_equal_to_plain_exclude_variation",
             "queries.tests.ExcludeTest17600.test_exclude_with_q_object_no_distinct",

django_mongodb/fields/auto.py

@@ -1,6 +1,7 @@
 from bson import ObjectId, errors
 from django.core import exceptions
 from django.db.models.fields import AutoField, Field
+from django.utils.functional import cached_property
 from django.utils.translation import gettext_lazy as _
 
 
@@ -15,14 +16,26 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     def get_prep_value(self, value):
-        # Override AutoField casting to integer.
-        return Field.get_prep_value(self, value)
+        if value is None:
+            return None
+        # Accept int for compatibility with Django's test suite which has many
+        # instances of manually assigned integer IDs, as well as for things
+        # like settings.SITE_ID which has a system check requiring an integer.
+        if isinstance(value, (ObjectId | int)):
+            return value
+        try:
+            return ObjectId(value)
+        except errors.InvalidId as e:
+            # A manually assigned integer ID?
+            if isinstance(value, str) and value.isdigit():
+                return int(value)
+            raise ValueError(f"Field '{self.name}' expected an ObjectId but got {value!r}.") from e
 
     def rel_db_type(self, connection):
         return Field().db_type(connection=connection)
 
     def to_python(self, value):
-        if value is None:
+        if value is None or isinstance(value, int):
             return value
         try:
             return ObjectId(value)
@@ -32,3 +45,8 @@ def to_python(self, value):
                 code="invalid",
                 params={"value": value},
             ) from None
+
+    @cached_property
+    def validators(self):
+        # Avoid IntegerField validators inherited from AutoField.
+        return [*self.default_validators, *self._validators]