Hello all,
I'll illustrate my problem with a small example.
Let's consider a data model:
--------------------------------------------------------------
from django.db import models
class OldTag(models.Model):
content = models.TextField()
class NewTag(models.Model):
content = models.TextField()
oldtags = models.ManyToManyField(OldTag)
class Document(models.Model):
content = models.TextField()
oldtags = models.ManyToManyField(OldTag)
--------------------------------------------------------------
So there are two many-to-many relationships between classes.
I'd like to find all the documents that are related to a specific new
tag. To reach the new tags, I need to pass old tags first, do I'd expect
to use something like this:
>>> Document.objects.filter(oldtags__newtags__content = 'popcorn')
But it doesn't work:
TypeError: Cannot resolve keyword 'newtags' into field
Let's consider an example data:
--------------------------------------------------------------
from myproject.myapp.models import OldTag, NewTag, Document
d, dc = Document.objects.get_or_create(content = "spam")
d.save()
ot, otc = OldTag.objects.get_or_create(content = "eggs")
ot.save()
nt, ntc = NewTag.objects.get_or_create(content = "popcorn")
nt.save()
d.oldtags.add(ot)
nt.oldtags.add(ot)
--------------------------------------------------------------
I can easily find all the documents with the specified old tag:
>>> Document.objects.filter(oldtags__content = 'eggs')
To find the field that I could access with the filter, I look inside the
old tag object:
>>> dir(ot)
['AddManipulator', 'ChangeManipulator', 'DoesNotExist', '__class__',
'__delattr__', '__dict__', '__doc__', '__eq__', '__getattribute__',
'__hash__', '__init__', '__metaclass__', '__module__', '__ne__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
'__str__', '__weakref__', '_collect_sub_objects', '_default_manager',
'_get_FIELD_display', '_get_FIELD_filename', '_get_FIELD_height',
'_get_FIELD_size', '_get_FIELD_url', '_get_FIELD_width',
'_get_image_dimensions', '_get_next_or_previous_by_FIELD',
'_get_next_or_previous_in_order', '_get_pk_val', '_meta', '_prepare',
'_save_FIELD_file', '_set_related_many_to_many', 'add_to_class',
'content', 'delete', 'document_set', 'id', 'newtag_set', 'objects',
'save', 'validate']
So there's something called newtag_set in the old tag object. It allows
to fetch all the new tags related to the old tag. I managed to get what
I want with this expression:
>>> Document.objects.filter(oldtags__in = NewTag.objects.get(content =
>>> 'popcorn').oldtags.all())
...but it executes two SQL queries, while it can be done with just one
query. Do you have any better ideas?
Regards,
Maciej
--
Maciej Bliziński
http://automatthias.wordpress.com
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