I should have pointed out that I got a NoSuchColumnError because of
"anon_1.anon_2". There is no column "anon_2" in any of the tables. It's
just an alias name of a derived table.
Is "table_name_1.table_name_2" supposed to mean anything?
On Tuesday, February 28, 2012 5:53:42 PM UTC+2, Michael Bayer wrote:
>
>
> On Feb 28, 2012, at 9:40 AM, naktinis wrote:
>
> Column "anon_1.anon_2" is generated in the following scenario:
>
> dbsession.query(FirstThing, FirstThing.moved_by.any(User.id ==
> user_id)).options(joinedload_all('some_property'))
> query = query.join(SecondThing, SecondThing.first_thing_id ==
> FirstThing.id)
> query = query.order_by(OneThing.ordering_field).limit(count)
>
> Also, it is important that both FirstThing and SecondThing polymorphically
> inherit from Thing.
>
> Effectively, query.all() generates a query like
>
> SELECT ... anon_1.anon_2 AS anon_1_anon_2 ...
> FROM
> (SELECT first_thing.id AS first_thing.id, EXISTS (SELECT 1 FROM
> first_thing_moves, users ...) AS anon_2
> FROM thing JOIN first_thing ON ... JOIN (SELECT ... FROM thing JOIN
> second_thing) AS anon_3 ON ... ORDER BY ... LIMIT ...) AS anon_1 ORDER BY
> ...
>
> Why would "anon_1.anon_2" column be generated there - it is, I think, not
> even a valid syntax?
>
>
> it's valid, "anon_1" is the label applied to a subquery, you can see where
> it has "(SELECT .... ) AS anon_1". "anon_1" becomes what we sometimes call
> a "derived table" in the query and is then valid like any other alias name.
>
> The join is because when we have a joined inheritance class B inherits
> from A, then we join to it from C, we are effectively joining:
>
> SELECT * FROM C JOIN (A JOIN B ON A.id=B.id) ON C.x=A.y
>
> That is valid SQL, however, it doesn't work on SQLite, and also doesn't
> work on MySQL versions before 5. It also may or may not have issues on
> some other backends. So SQLAlchemy turns "A JOIN B" into a subquery:
>
> SELECT * FROM C JOIN (SELECT * FROM A JOIN B ON A.id=B.id) AS anon_1 ON
> C.x=anon_1.y
>
> as it turns out, this approach generalizes much more nicely than just
> putting "A JOIN B" in there. Suppose classes B1 and B2 inherit from A in a
> concrete fashion, using tables "B1" and "B2" to represent the full row.
> Then you wanted to join from C to A. SQLAlchemy would have you doing a
> "polymorphic union" which means you select from the UNION of B1 and B2:
>
> SELECT * FROM C JOIN (SELECT * FROM B1 UNION SELECT * FROM B2) AS anon_1
> ON C.x=anon_1.y
>
> where "anon_1.y" here would be "y" from B1 unioned to "y" from B2.
>
> Anyway, SQLAlchemy is very quick to wrap up a series of rows in a
> subquery, applying an alias to it, since that syntax works the most
> consistently across not only all backends but across a really wide range of
> scenarios.
>
>
>
>
>
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