On 9/6/07, omat <[EMAIL PROTECTED]> wrote: > > Hi, > > For Question & Answer models: > > Question.objects.filter(answer__isnull=False) > > returns the set of questions that has at least one answer, but: > > Question.objects.filter(answer__isnull=True) > > does not return the questions that has no answer. > > Am I missing something?
Somewhat. The problem is that 'isnull' isn't doing what you think it does. When you run Question.objects.filter(answer__isnull=True), this gets turned into an SQL query something like: SELECT Question.* from Question INNER JOIN Answer WHERE Question.id = Answer.question_id WHERE Answer.id IS NULL In evaluating this query, this generates an internal table containing the columns: Question.id Question.field1 Question.field2, ... Answer.id Answer.field1 Answer.field2 This internal table is then truncated, and only the Question columns are returned to create Question Django objects. This internal table will contain a row for every Question+Answer pair where the answer points at the question. If there is no answer to join with a question, then there will be no answer in the result set. Hence, when you ask for answer__isnull=False - you get all the rows that are returned. If you ask for answer__isnull=True, you get nothing - because there can't be an answer-null row that is joined with a Question. Now - I'm sure the next question is "how to I get all questions that have at least one answer". The best SQL way to do this is with an aggregate clause, exploiting GROUP BY and HAVING to establish how many joined objects exist. However, Django doesn't currently have good support for aggregate clauses (something I'm hoping to rectify soon). An alternate approach is to do it in two steps: get a list of all answers, find the unique question ids (using distinct() and values() clauses), and then ask for Question.objects.filter(id__in=[list of ids]), or Question.objects.exclude(id__in=[list of ids]). Yours, Russ Magee %-) --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
