Hi,
I am having a hard time with Q objects based on subqueries. It's a
part of a linguistic corpus analysis query builder (hence Q objects as
this is how I parse and translate arbitrary queries into Django) --
the user can search 'posts' that meet complex criteria. Anyway, it's
all working fine and dandy with the exception of querying post tags
(there is a many-to-many relationship between posts and tags).
A simplified version of my models.py:
--- BEGIN ---
class Tag(models.Model):
label = models.CharField(max_length=255)
class Post(models.Model):
tags = models.ManyToManyField(Tag, through='TagsForPost')
class TagsForPost(models.Model):
tag = models.ForeignKey(Tag)
post = models.ForeignKey(Post)
--- END ---
The query parser iterates over the query expression and for every
criterion (=smallest subexpression) it adds a Q object. Let's say I
have this query:
'Tag' 'is' "tagA"
AND
'Tag' 'is' "tagB"
which should yield all posts that are tagged with both tagA and tagB.
So I feel I need something that will generate IN operator in SQL, with
the condition set being the result of a subquery fetching all tags for
a post. This seems to be handled nicely by Django:
http://docs.djangoproject.com/en/dev/ref/models/querysets/#in
However, since I am looking for posts, I need to cross-reference the
post in the inner query with the post in the outer query (on the SQL
level at least). I guess matching posts by primary key is the way to
go here. However,
I can't quite wrap my mind around the Django syntax here. Can anyone
help me with getting the proper Q object that will do the trick?
Let's assume that for every iteration the tag name (string) is held in
a variable called 'value'. As an example, for conditions for searching
for posts starting with a given text I do:
result = Q(body_anonymized__istartswith=value)
('body' was originally a part of Post above in models.py)
Thanks in advance!
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