Spanning relationship to obtain field value with .annotate() and .values() clauses - django

Question

I'm making some computations using values & annotate on queryset. Let's consider this model:
class Foo(models.Model):
fk_bar = models.ForeignKey(to=Bar, ....)
foo_val = models.IntegerField(...)
class Bar(models.Model):
attr = models.Charfield(...)
val = models.IntegerField(...)
So I can do:
Foo.object.all().values("fk_bar")
in order to group by the foreign relationship (some Foo might point to the same Bar).
Then I can do
Foo.object.all().values("fk_bar").annotate(qte=Sum("foo_val"))
To the the sum of foo_val for all the object with the same fk_bar, which yields something like:
{"fk_bar":<int>, "qte": <int>}
However I want the resulting dictionnary to calso contain Bar.attr, e.g. something like:
Foo.object.all().values("fk_bar").annotate(qte=Sum("foo_val")).annotate(bar_attr="fk_bar__attr")
To get something like:
{"fk_bar":<int>, "qte": <int>, "bar_attr":<str>}
However that fails (TypeError: Queryset.annotate() received a non-expression). Any ways to go around this?

Answer 1

One option is to specify the additional values to "keep" from before the aggregation in the values() clause, like so:
Foo.object.all().values("fk_bar", "fk_bar__attr").annotate(qte=Sum("foo_val"))
Which will yield roughly what's asked.
In general, if one puts more fields in the .values() clause, those fields will be available the in the output dictionary. Any "additional" fields (e.g. not from the raw queryset data) that needs to be computed beforehand (e.g. before aggregation) can be "created" with .annotation() (like qte=Sum(...) above), and then included in the subsequent .values("fk_bar", "qte", ...). A few examples:
Yields the sum for all the Foos with the same fk_bar:
Foo.object.all().values("fk_bar").annotate(qte=Sum("foo_val"))
This will group all the Foos by fk_bar, by foo_val. Then for each group, will sum up the values of the fk_bar__val field:
Foo.object.all().values("fk_bar", "foo_val").annotate(qte=Sum("fk_bar__val"))
The thing to note is that if many fields are specified in .values(), then the aggregation will be larger (e.g. 1 dictionnary for each unique combination of values).