AFAIK there is no direct way to do this sort of thing in a single
query. select_related will only get foreign key relations into the
query so the ORM will always do an SQL query for each row in your many
to many. In these types of situations I tend to select everything I
need and then run some sort of pre-processing to put the two sets
together, thus ensuring only 2 queries rather than n+1 (where n is the
number of items in your original queryset).
Scott Gould's answer is about as good as you'll get I reckon.
On Jun 21, 3:37 pm, JeffH <holtzma...@gmail.com> wrote:
> That looks reasonable... but I wonder if the ORM can do it directly
> somehow. Anyone?
>
> On Jun 21, 10:14 am, Scott Gould <zinck...@gmail.com> wrote:
>
> > There may well be a better way to do this, especially since it's been
> > a good year since I was struggling with this myself. (Very similar
> > case to yours, different subject matter of course.)
>
> > The way I ended up doing it was to use a template tag and some list
> > comprehensions to whittle things down. E.g.:
>
> > questions = Questions.objects.all()
> > answers = Answers.objects.filter(candidate=my_candidate)
>
> > questions_and_answers = [(q, [a for a in answers if a.question = q])
> > for q in questions]
>
> > ...which should give you a list of (question, <list of answers>)
> > tuples.
>
> > On Jun 21, 10:00 am, JeffH <holtzma...@gmail.com> wrote:
>
> > > To clarify: Each race has a set of questions. The candidate may have
> > > responded to none, some, or all. The answers are linked to the
> > > candidate (and to the question). For each candidate, I want to display
> > > all the questions, with or without answer. The way it works currently,
> > > only the questions with answers get displayed.
>
> > > On Jun 21, 8:41 am, Daniel Roseman <dan...@roseman.org.uk> wrote:
>
> > > > On Jun 21, 12:51 pm, JeffH <holtzma...@gmail.com> wrote:
>
> > > > > I have some models that (simplified) look like the following.
>
> > > > > class Answer(models.Model):
> > > > > id = models.CharField(max_length=32, primary_key=True)
> > > > > text = models.TextField(blank=False)
> > > > > question = models.ForeignKey(Question)
> > > > > candidate = models.ForeignKey(Candidate)
>
> > > > > class Question(models.Model):
> > > > > id = models.CharField(max_length=32, primary_key=True)
> > > > > text = models.TextField(blank=False)
>
> > > > > class Candidate(models.Model):
> > > > > id = models.CharField(max_length=32, primary_key=True)
> > > > > name = models.CharField(max_length=32, blank=False)
>
> > > > > class Race(models.Model):
> > > > > id = models.CharField(max_length=32, primary_key=True)
> > > > > name = models.CharField(max_length=128, blank=False)
> > > > > questions = models.ManyToManyField(Question)
> > > > > candidates = models.ManyToManyField(Candidate)
>
> > > > > So, a Race has Candidates and Questions, and a Candidate has Answers.
> > > > > Each answer is associated with a Question and a Candidate. Displaying
> > > > > the question associated with an answer is easy:
>
> > > > > # context variable in view
> > > > > answers = Answer.objects.filter(candidate=candidate)
>
> > > > > # template code
> > > > > <table>
> > > > > {% for answer in answers %}
> > > > > <tr>
> > > > > <td>{{answer.question.text}}</td>
> > > > > <td>{{answer.text}}</td>
> > > > > </tr>
> > > > > {% endfor %}
> > > > > </table>
>
> > > > > From the point of view of the Candidate, I need to display all the
> > > > > questions, including the ones without Answers. I know how to to do
> > > > > this using raw sql and an outer join. How to do it in the orm?
>
> > > > > Thanks in advance for any ideas.
>
> > > > > --Jeff
>
> > > > Not quite enough information here to answer. What are you wanting to
> > > > join? If you just want to display all the questions, why do you need a
> > > > join at all?
> > > > --
> > > > DR.
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