How can I get computed elements of a table in a django queryset?

Question

I\'m trying to use django\'s queryset API to emulate the following query:

SELECT EXTRACT(year FROM chosen_date) AS year, 
EXTRACT(month FROM chosen_date) AS          


        

Answer 1

Short answer: If you create an aliased (or computed) column using extra(select=...) then you cannot use the aliased column in a subsequent call to filter(). Also, as you've discovered, you can't use the aliased column in later calls to extra(select=...) or extra(where=...).

An attempt to explain why:

For example:

qs = MyModel.objects.extra(select={'alias_col': 'title'})

#FieldError: Cannot resolve keyword 'alias_col' into field...
filter_qs = qs.filter(alias_col='Camembert')

#DatabaseError: column "alias_col" does not exist
extra_qs = qs.extra(select={'another_alias': 'alias_col'})

filter_qs will try to produce a query like:

SELECT (title) AS "alias_col", "myapp_mymodel"."title"
FROM "myapp_mymodel"
WHERE alias_col = "Camembert";

And extra_qs tries something like:

SELECT (title) AS "alias_col", (alias_col) AS "another_alias",
        "myapp_mymodel"."title"
FROM "myapp_mymodel";

Neither of those is valid SQL. In general, if you want to use a computed column's alias multiple times in the SELECT or WHERE clauses of query you actually need to compute it each time. This is why Roman Pekar's answer solves your specific problem - instead of trying to compute chosen_date once and then use it again later he computes it each time it's needed.

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You mention Annotation/Aggregation in your question. You can use filter() on the aliases created by annotate() (so I'd be interested in seeing the similar errors you're talking about, it's been fairly robust in my experience). That's because when you try to filter on an alias created by annotate, the ORM recognizes what you're doing and replaces the alias with the computation that created it.

So as an example:

qs = MyModel.objects.annotate(alias_col=Max('id'))
qs = qs.filter(alias_col__gt=0)

Produces something like:

SELECT "myapp_mymodel"."id", "myapp_mymodel"."title",
        MAX("myapp_mymodel"."id") AS "alias_col"
FROM "myapp_mymodel"
GROUP BY "myapp_mymodel"."id", "myapp_mymodel"."title"
HAVING MAX("myapp_mymodel"."id") > 0;

Using "HAVING MAX alias_col > 0" wouldn't work.

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I hope that's helpful. If there's anything I've explained badly let me know and I'll see if I can improve it.