Add len lookup

Author: WaVEV

django_mongodb_backend/fields/array.py

@@ -338,7 +338,7 @@ class ArrayLenTransform(Transform):
 
     def as_mql(self, compiler, connection):
         lhs_mql = process_lhs(self, compiler, connection)
-        return {"$cond": {"if": {"$eq": [lhs_mql, None]}, "then": None, "else": {"$size": lhs_mql}}}
+        return {"$cond": {"if": {"$isArray": lhs_mql}, "then": {"$size": lhs_mql}, "else": None}}
 
 
 @ArrayField.register_lookup

tests/model_fields_/test_embedded_model.py

@@ -256,6 +256,28 @@ def test_filter_with_embeddedfield_array_path(self):
             [self.lost_empires],
         )
 
+    def test_len(self):
+        self.assertCountEqual(MuseumExhibit.objects.filter(sections__len=10), [])
+        self.assertCountEqual(
+            MuseumExhibit.objects.filter(sections__len=1), [self.egypt, self.new_descoveries]
+        )
+        # Nested EMF
+        self.assertCountEqual(
+            MuseumExhibit.objects.filter(main_section__artifacts__len=1), [self.lost_empires]
+        )
+        self.assertCountEqual(MuseumExhibit.objects.filter(main_section__artifacts__len=2), [])
+        self.assertCountEqual(MuseumExhibit.objects.filter(main_section__artifacts__len=2), [])
+        # Nested Indexed Array
+        self.assertCountEqual(
+            MuseumExhibit.objects.filter(sections__0__artifacts__len=2), [self.wonders]
+        )
+        self.assertCountEqual(
+            MuseumExhibit.objects.filter(sections__0__artifacts__len=0), [self.new_descoveries]
+        )
+        self.assertCountEqual(
+            MuseumExhibit.objects.filter(sections__1__artifacts__len=1), [self.wonders]
+        )
+
 
 class QueryingTests(TestCase):
     @classmethod