make DurationField store milliseconds instead of microseconds

MongoDB's arithmetic operators like $add treat numbers are milliseconds.

Author: timgraham

README.md

@@ -118,7 +118,8 @@ Migrations for 'admin':
 
 - Queries with joins aren't supported.
 
-- `DateTimeField` doesn't support microsecond precision.
+- `DateTimeField` doesn't support microsecond precision, and correspondingly,
+  `DurationField` stores milliseconds rather than microseconds.
 
 - The following database functions aren't supported:
     - `Chr`

django_mongodb/expressions.py

@@ -70,6 +70,9 @@ def value(self, compiler, connection):  # noqa: ARG001
     elif isinstance(value, datetime.date):
         # Turn dates into datetimes since BSON doesn't support dates.
         value = datetime.datetime.combine(value, datetime.datetime.min.time())
+    elif isinstance(value, datetime.timedelta):
+        # DurationField store milliseconds rather than microseconds.
+        value = value / datetime.timedelta(millisecond=1)
     return {"$literal": value}
 
 

django_mongodb/fields/__init__.py

@@ -1,8 +1,10 @@
 from .auto import MongoAutoField
+from .duration import register_duration_field
 from .json import register_json_field
 
 __all__ = ["register_fields", "MongoAutoField"]
 
 
 def register_fields():
+    register_duration_field()
     register_json_field()

django_mongodb/fields/duration.py

@@ -0,0 +1,15 @@
+from django.db.models.fields import DurationField
+
+_get_db_prep_value = DurationField.get_db_prep_value
+
+
+def get_db_prep_value(self, value, connection, prepared=False):
+    """DurationField stores milliseconds rather than microseconds."""
+    value = _get_db_prep_value(self, value, connection, prepared)
+    if connection.vendor == "mongodb" and value is not None:
+        value = value // 1000
+    return value
+
+
+def register_duration_field():
+    DurationField.get_db_prep_value = get_db_prep_value

django_mongodb/operations.py

@@ -101,6 +101,11 @@ def convert_decimalfield_value(self, value, expression, connection):
             value = value.to_decimal()
         return value
 
+    def convert_durationfield_value(self, value, expression, connection):
+        if value is not None:
+            value = datetime.timedelta(milliseconds=value)
+        return value
+
     def convert_jsonfield_value(self, value, expression, connection):
         """
         Convert dict data to a string so that JSONField.from_db_value() can