Allow using Extract() with DurationField

[Daniel Hahler]

I am trying to use Extract on a duration field, but this results in ValueError: Extract input expression must be DateField, DateTimeField, or TimeField.:

qs = qs.annotate(
    speed=Sum(
        F('distance_in_meters') * Value(3.6) /
            Extract(F('duration_in_seconds'), 'epoch'),
        output_field=FloatField()),
)

The following patch however makes it work on PostgreSQL, so it should be considered to be enabled there - in case other DB backends do not support it:

diff --git i/django/db/models/functions/datetime.py w/django/db/models/functions/datetime.py
index 2980460..3266ef6 100644
--- i/django/db/models/functions/datetime.py
+++ w/django/db/models/functions/datetime.py
@@ -5,6 +5,7 @@
 from django.conf import settings
 from django.db.models import (
     DateField, DateTimeField, IntegerField, TimeField, Transform,
+    DurationField,
 )
 from django.db.models.lookups import (
     YearExact, YearGt, YearGte, YearLt, YearLte,
@@ -52,6 +53,8 @@ def as_sql(self, compiler, connection):
             sql = connection.ops.date_extract_sql(self.lookup_name, sql)
         elif isinstance(lhs_output_field, TimeField):
             sql = connection.ops.time_extract_sql(self.lookup_name, sql)
+        elif isinstance(lhs_output_field, DurationField):
+            sql = connection.ops.date_extract_sql(self.lookup_name, sql)
         else:
             # resolve_expression has already validated the output_field so this
             # assert should never be hit.
@@ -61,7 +64,7 @@ def as_sql(self, compiler, connection):
     def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
         copy = super(Extract, self).resolve_expression(query, allow_joins, reuse, summarize, for_save)
         field = copy.lhs.output_field
-        if not isinstance(field, (DateField, DateTimeField, TimeField)):
+        if not isinstance(field, (DateField, DateTimeField, TimeField, DurationField)):
             raise ValueError('Extract input expression must be DateField, DateTimeField, or TimeField.')
         # Passing dates to functions expecting datetimes is most likely a mistake.
         if type(field) == DateField and copy.lookup_name in ('hour', 'minute', 'second'):

Am I missing something?
What needs to be done to get it enabled for DurationFields?

[Tim Graham]

I'm not sure, Josh might have some advice. If we have a tested patch, we can go from there.

[Daniel Hahler]

Cool, working on a PR now.

I've noticed that with SQLite, the "Extract" for a DurationField appears to return NULL/None always, e.g. with DTModel.objects.annotate(duration_s=Extract('duration', 'second')).

Should this throw some error instead? Would that be possible in Extract already (so basically keeping the current behavior there for unsupported backends)?
Or would it need some other special casing?

[Daniel Hahler]

​https://github.com/django/django/pull/7600

[Tim Graham <timograham@…>]

In 43a4835e:

Fixed #27473 -- Added DurationField support to Extract.

[Jurgis Pralgauskis]

Are such features considered for backporting to 1.11?

[Claude Paroz]

Not at all, 1.11 only receive security updates.