Some queries relabel the table name with an alias, but aggregate-if can't follow it properly.
>>> qs = Book.objects.annotate(
... mean_age=Avg('authors__age'))
... mean_age2=Avg('authors__age', only=Q(authors__age__gte=0))
... )
>>> print qs
*** DatabaseError: no such column: aggregation_author.age
The generated query obtained by print books.query is:
SELECT
"aggregation_book"."id", "aggregation_book"."isbn",
"aggregation_book"."name", "aggregation_book"."pages",
"aggregation_book"."rating", "aggregation_book"."price",
"aggregation_book"."contact_id", "aggregation_book"."publisher_id",
"aggregation_book"."pubdate", AVG(T5."age") AS "mean_age",
AVG(CASE WHEN "aggregation_author"."age" >= 0
THEN T5."age" ELSE null END) AS "mean_age2"
FROM
"aggregation_book"
LEFT OUTER JOIN "aggregation_book_authors"
ON ("aggregation_book"."id" = "aggregation_book_authors"."book_id")
LEFT OUTER JOIN "aggregation_author" T5
ON ("aggregation_book_authors"."author_id" = T5."id")
GROUP BY
"aggregation_book"."id", "aggregation_book"."isbn",
"aggregation_book"."name", "aggregation_book"."pages",
"aggregation_book"."rating", "aggregation_book"."price",
"aggregation_book"."contact_id", "aggregation_book"."publisher_id",
"aggregation_book"."pubdate";
The problem is that the WHEN expression generated by the only argument is using the table name aggregation_author instead of T5.
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