Union and Intersect in Django

Question

class Tag(models.Model):   name = models.CharField(maxlength=100)  class Blog(models.Model):   name = models.CharField(maxlength=100)   tags =  models.ManyToManyField(Tag) 

Simple models just to ask my question.

I wonder how can i query blogs using tags in two different ways.

• Blog entries that are tagged with "tag1" or "tag2": Blog.objects.filter(tags_in=[1,2]).distinct()
• Blog objects that are tagged with "tag1" and "tag2" : ?
• Blog objects that are tagged with exactly "tag1" and "tag2" and nothing else : ??

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Tag and Blog is just used for an example.

Answer 1

You could use Q objects for #1:

# Blogs who have either hockey or django tags. from django.db.models import Q Blog.objects.filter(     Q(tags__name__iexact='hockey') | Q(tags__name__iexact='django') ) 

Unions and intersections, I believe, are a bit outside the scope of the Django ORM, but its possible to to these. The following examples are from a Django application called called django-tagging that provides the functionality. Line 346 of models.py:

For part two, you're looking for a union of two queries, basically

def get_union_by_model(self, queryset_or_model, tags):     """     Create a ``QuerySet`` containing instances of the specified     model associated with *any* of the given list of tags.     """     tags = get_tag_list(tags)     tag_count = len(tags)     queryset, model = get_queryset_and_model(queryset_or_model)      if not tag_count:         return model._default_manager.none()      model_table = qn(model._meta.db_table)     # This query selects the ids of all objects which have any of     # the given tags.     query = """     SELECT %(model_pk)s     FROM %(model)s, %(tagged_item)s     WHERE %(tagged_item)s.content_type_id = %(content_type_id)s       AND %(tagged_item)s.tag_id IN (%(tag_id_placeholders)s)       AND %(model_pk)s = %(tagged_item)s.object_id     GROUP BY %(model_pk)s""" % {         'model_pk': '%s.%s' % (model_table, qn(model._meta.pk.column)),         'model': model_table,         'tagged_item': qn(self.model._meta.db_table),         'content_type_id': ContentType.objects.get_for_model(model).pk,         'tag_id_placeholders': ','.join(['%s'] * tag_count),     }      cursor = connection.cursor()     cursor.execute(query, [tag.pk for tag in tags])     object_ids = [row[0] for row in cursor.fetchall()]     if len(object_ids) > 0:         return queryset.filter(pk__in=object_ids)     else:         return model._default_manager.none() 

For part #3 I believe you're looking for an intersection. See line 307 of models.py

def get_intersection_by_model(self, queryset_or_model, tags):     """     Create a ``QuerySet`` containing instances of the specified     model associated with *all* of the given list of tags.     """     tags = get_tag_list(tags)     tag_count = len(tags)     queryset, model = get_queryset_and_model(queryset_or_model)      if not tag_count:         return model._default_manager.none()      model_table = qn(model._meta.db_table)     # This query selects the ids of all objects which have all the     # given tags.     query = """     SELECT %(model_pk)s     FROM %(model)s, %(tagged_item)s     WHERE %(tagged_item)s.content_type_id = %(content_type_id)s       AND %(tagged_item)s.tag_id IN (%(tag_id_placeholders)s)       AND %(model_pk)s = %(tagged_item)s.object_id     GROUP BY %(model_pk)s     HAVING COUNT(%(model_pk)s) = %(tag_count)s""" % {         'model_pk': '%s.%s' % (model_table, qn(model._meta.pk.column)),         'model': model_table,         'tagged_item': qn(self.model._meta.db_table),         'content_type_id': ContentType.objects.get_for_model(model).pk,         'tag_id_placeholders': ','.join(['%s'] * tag_count),         'tag_count': tag_count,     }      cursor = connection.cursor()     cursor.execute(query, [tag.pk for tag in tags])     object_ids = [row[0] for row in cursor.fetchall()]     if len(object_ids) > 0:         return queryset.filter(pk__in=object_ids)     else:         return model._default_manager.none()