Hi there. I'm running into some strange behavior when using `defer` with
`select_related` and I wanted to find out if I'm simply misunderstanding
what should be going on, or if indeed I have stumbled onto a bug.
Summary: Using `defer` on a table that has been re-included in a query via
`select_related` affects the fields returned by "original" table.
Consider in the following example that each `Company` might have many
users, and each company might have been created by a user owned by the
company itself or a different company. Now we'd like to query the database
for a list of companies, along with enough information about the `creator`
to display, say, their name and company name.
(For the sake of the example the number of fields is very small, but in
practice I ran into this bug because doing multiple `select_related`s on a
wide table actually caused me to hit the 1664 column limit in Postgres).
# This is in an app "defer".
from django.db import models
class Company(models.Model):
creator = models.ForeignKey('defer.User', null=True)
name = models.CharField(max_length=32)
description = models.TextField()
class User(models.Model):
company = models.ForeignKey(Company)
username = models.CharField(max_length=32)
name = models.CharField(max_length=32)
Now we query `Company` in such a way that it is `select_related` again into
the query, and defer fields on the instance of the table that is
`select_related`, but not on the "main" instance of the table.
>>> cs = Company.objects.all().select_related(
'creator',
'creator__company').defer('creator__company__description')
>>>cs.first().description # Causes a query.
Printing the SQL shows what's going wrong:
>>> print cs.query
SELECT "defer_company"."id", "defer_company"."creator_id",
"defer_company"."name", "defer_user"."id",
"defer_user"."company_id", "defer_user"."username",
"defer_user"."name",
T3."id", T3."creator_id", T3."name"
FROM "defer_company"
LEFT OUTER JOIN "defer_user" ON ( "defer_company"."creator_id" =
"defer_user"."id" )
LEFT OUTER JOIN "defer_company" T3 ON ( "defer_user"."company_id" =
T3."id" )
Note that we are *not* retrieving the field `defer_company.description`. It
seems that the `defer` called on the instance of the `defer_company` table
selected through the `creator` field (`T3` above) is being applied to the
"first" instance of the table as well.
Updating the SQL to include that column (in the way I expected `defer` and
`select_related` to behave) succeeds in returning the correct data -- in
sqlite3, at least.
I encountered this behavior in version 1.6.5 and also reproduced it in
1.7c2.
Thanks!
-Zach
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