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 because of that,
+  `DurationField` stores milliseconds rather than microseconds.
 
 - The following database functions aren't supported:
     - `Chr`

django_mongodb/__init__.py

@@ -7,11 +7,13 @@
 check_django_compatability()
 
 from .expressions import register_expressions  # noqa: E402
+from .fields import register_fields  # noqa: E402
 from .functions import register_functions  # noqa: E402
 from .lookups import register_lookups  # noqa: E402
 from .query import register_nodes  # noqa: E402
 
 register_expressions()
+register_fields()
 register_functions()
 register_lookups()
 register_nodes()

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.total_seconds() * 1000
     return {"$literal": value}
 
 

django_mongodb/fields/__init__.py

@@ -1,3 +1,8 @@
 from .auto import MongoAutoField
+from .duration import register_duration_field
 
-__all__ = ["MongoAutoField"]
+__all__ = ["register_fields", "MongoAutoField"]
+
+
+def register_fields():
+    register_duration_field()

django_mongodb/fields/duration.py

@@ -0,0 +1,17 @@
+from decimal import Decimal
+
+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 = int(Decimal(value) / 1000)
+    return value
+
+
+def register_duration_field():
+    DurationField.get_db_prep_value = get_db_prep_value

django_mongodb/operations.py

@@ -89,6 +89,13 @@ 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:
+            # Multiplying value converts milliseconds stored in the database
+            # to microseconds expected by timedelta.
+            value = datetime.timedelta(0, 0, value * 1000)
+        return value
+
     def convert_timefield_value(self, value, expression, connection):
         if value is not None:
             value = value.time()