Add ObjectIdField unit tests.

tests/model_fields_/test_objectidfield.py

@@ -0,0 +1,23 @@
+from bson import ObjectId
+from django.test import SimpleTestCase
+
+from django_mongodb.fields import ObjectIdField
+
+
+class MethodTests(SimpleTestCase):
+    def test_deconstruct(self):
+        field = ObjectIdField()
+        name, path, args, kwargs = field.deconstruct()
+        self.assertEqual(path, "django_mongodb.fields.ObjectIdField")
+        self.assertEqual(args, [])
+        self.assertEqual(kwargs, {})
+
+    def test_get_internal_type(self):
+        f = ObjectIdField()
+        self.assertEqual(f.get_internal_type(), "ObjectIdField")
+
+    def test_to_python(self):
+        f = ObjectIdField()
+        expected = ObjectId("1" * 24)
+        self.assertEqual(f.to_python("1" * 24), expected)
+        self.assertEqual(f.to_python(expected), expected)