diff --git a/django_mongodb/features.py b/django_mongodb/features.py
index e3cc4aee1..6236ed770 100644
--- a/django_mongodb/features.py
+++ b/django_mongodb/features.py
@@ -33,9 +33,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     uses_savepoints = False
 
     _django_test_expected_failures = {
-        # Database defaults not supported: bson.errors.InvalidDocument:
-        # cannot encode object: <django.db.models.expressions.DatabaseDefault
-        "basic.tests.ModelInstanceCreationTests.test_save_primary_with_db_default",
         # 'NulledTransform' object has no attribute 'as_mql'.
         "lookup.tests.LookupTests.test_exact_none_transform",
         # "Save with update_fields did not affect any rows."
@@ -93,7 +90,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "schema.tests.SchemaTests.test_order_index",
         "schema.tests.SchemaTests.test_text_field_with_db_index",
         # AlterField
-        "schema.tests.SchemaTests.test_alter",
         "schema.tests.SchemaTests.test_alter_auto_field_to_integer_field",
         "schema.tests.SchemaTests.test_alter_field_add_index_to_integerfield",
         "schema.tests.SchemaTests.test_alter_field_default_dropped",
@@ -104,13 +100,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "schema.tests.SchemaTests.test_alter_int_pk_to_int_unique",
         "schema.tests.SchemaTests.test_alter_not_unique_field_to_primary_key",
         "schema.tests.SchemaTests.test_alter_null_to_not_null",
-        "schema.tests.SchemaTests.test_alter_null_to_not_null_keeping_default",
         "schema.tests.SchemaTests.test_alter_primary_key_the_same_name",
         "schema.tests.SchemaTests.test_autofield_to_o2o",
-        # AlterField (rename)
-        "schema.tests.SchemaTests.test_rename",
-        "schema.tests.SchemaTests.test_rename_keep_db_default",
-        "schema.tests.SchemaTests.test_rename_keep_null_status",
         # AlterField (db_index)
         "schema.tests.SchemaTests.test_indexes",
         "schema.tests.SchemaTests.test_remove_constraints_capital_letters",
@@ -125,15 +116,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # alter_unique_together
         "schema.tests.SchemaTests.test_remove_unique_together_does_not_remove_meta_constraints",
         "schema.tests.SchemaTests.test_unique_together",
-        # ManyToManyField
-        "schema.tests.SchemaTests.test_m2m_rename_field_in_target_model",
-        "schema.tests.SchemaTests.test_m2m_repoint",
-        "schema.tests.SchemaTests.test_m2m_repoint_custom",
-        "schema.tests.SchemaTests.test_m2m_repoint_inherited",
-        "schema.tests.SchemaTests.test_m2m_through_alter",
-        "schema.tests.SchemaTests.test_m2m_through_alter_custom",
-        "schema.tests.SchemaTests.test_m2m_through_alter_inherited",
-        "schema.tests.SchemaTests.test_m2m_through_remove",
         # add/remove_constraint
         "schema.tests.SchemaTests.test_composed_constraint_with_fk",
         "schema.tests.SchemaTests.test_remove_ignored_unique_constraint_not_create_fk_index",
@@ -159,7 +141,11 @@ def django_test_expected_failures(self):
 
     django_test_skips = {
         "Database defaults aren't supported by MongoDB.": {
+            # bson.errors.InvalidDocument: cannot encode object:
+            # <django.db.models.expressions.DatabaseDefault
+            "basic.tests.ModelInstanceCreationTests.test_save_primary_with_db_default",
             "schema.tests.SchemaTests.test_db_default_output_field_resolving",
+            "schema.tests.SchemaTests.test_rename_keep_db_default",
         },
         "Insert expressions aren't supported.": {
             "bulk_create.tests.BulkCreateTests.test_bulk_insert_now",
diff --git a/django_mongodb/fields/auto.py b/django_mongodb/fields/auto.py
index c79a21e8b..1a8a9ee32 100644
--- a/django_mongodb/fields/auto.py
+++ b/django_mongodb/fields/auto.py
@@ -1,6 +1,6 @@
 from bson import ObjectId, errors
 from django.core import exceptions
-from django.db.models.fields import AutoField, Field
+from django.db.models.fields import AutoField
 from django.utils.functional import cached_property
 from django.utils.translation import gettext_lazy as _
 
@@ -31,8 +31,8 @@ def get_prep_value(self, value):
                 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 db_type(self, connection):
+        return "ObjectId"
 
     def to_python(self, value):
         if value is None or isinstance(value, int):
diff --git a/django_mongodb/schema.py b/django_mongodb/schema.py
index 896ad8dd2..5867a2365 100644
--- a/django_mongodb/schema.py
+++ b/django_mongodb/schema.py
@@ -27,8 +27,22 @@ def add_field(self, model, field):
                 {}, [{"$set": {column: self.effective_default(field)}}]
             )
 
-    def alter_field(self, model, old_field, new_field, strict=False):
-        pass
+    def _alter_field(
+        self,
+        model,
+        old_field,
+        new_field,
+        old_type,
+        new_type,
+        old_db_params,
+        new_db_params,
+        strict=False,
+    ):
+        # Have they renamed the column?
+        if old_field.column != new_field.column:
+            self.connection.database[model._meta.db_table].update_many(
+                {}, {"$rename": {old_field.column: new_field.column}}
+            )
 
     def remove_field(self, model, field):
         # Remove implicit M2M tables.