implement array length

Author: timgraham

django_mongodb/features.py

@@ -94,17 +94,17 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # int() argument must be a string, a bytes-like object or a real number, not 'list'
         "model_fields_.test_arrayfield.TestQuerying.test_index_annotation",
         # Wrong results
-        "model_fields_.test_arrayfield.TestQuerying.test_in_as_F_object",
         "model_fields_.test_arrayfield.TestQuerying.test_index",
         "model_fields_.test_arrayfield.TestQuerying.test_index_chained",
         "model_fields_.test_arrayfield.TestQuerying.test_index_nested",
-        "model_fields_.test_arrayfield.TestQuerying.test_len",
-        "model_fields_.test_arrayfield.TestQuerying.test_len_empty_array",
-        "model_fields_.test_arrayfield.TestQuerying.test_lt",
         "model_fields_.test_arrayfield.TestQuerying.test_order_by_slice",
         "model_fields_.test_arrayfield.TestQuerying.test_slice",
         "model_fields_.test_arrayfield.TestQuerying.test_slice_annotation",
-        "model_fields_.test_arrayfield.TestQuerying.test_usage_in_subquery",
+        # $lt treats null values as zero.
+        "model_fields_.test_arrayfield.TestQuerying.test_lt",
+        "model_fields_.test_arrayfield.TestQuerying.test_len",
+        # None is $in None
+        "model_fields_.test_arrayfield.TestQuerying.test_in_as_F_object",
     }
     # $bitAnd, #bitOr, and $bitXor are new in MongoDB 6.3.
     _django_test_expected_failures_bitwise = {

django_mongodb/fields/array.py

@@ -9,6 +9,7 @@
 
 from django_mongodb.forms import SimpleArrayField
 
+from ..query_utils import process_lhs
 from ..utils import prefix_validation_error
 
 __all__ = ["ArrayField"]
@@ -277,16 +278,8 @@ class ArrayLenTransform(Transform):
     output_field = IntegerField()
 
     def as_mql(self, compiler, connection):
-        lhs, params = compiler.compile(self.lhs)
-        # Distinguish NULL and empty arrays
-        return (
-            (
-                ""  # "CASE WHEN %(lhs)s IS NULL THEN NULL ELSE "
-                # "coalesce(array_length(%(lhs)s, 1), 0) END"
-            )
-            % {},
-            params * 2,
-        )
+        lhs_mql = process_lhs(self, compiler, connection)
+        return {"$cond": {"if": {"$eq": [lhs_mql, None]}, "then": None, "else": {"$size": lhs_mql}}}
 
 
 @ArrayField.register_lookup

tests/model_fields_/models.py

@@ -56,6 +56,9 @@ class NullableIntegerArrayModel(models.Model):
     field_nested = ArrayField(ArrayField(models.IntegerField(null=True)), null=True)
     order = models.IntegerField(null=True)
 
+    def __str__(self):
+        return str(self.field)
+
 
 class CharArrayModel(models.Model):
     field = ArrayField(models.CharField(max_length=10))