Re: Prefetch Related with Multiple Column Join

Hi Ram,

I don't have a complete solution for you but I suggest you take a look at the
django.db.models.fields.related.ForeignObject class.

If I'm not mistaken it should allow you to define multi-column relationships
on both your Article and PublisherMediumBilling models and refer to them using
the `select_related()` and `prefetch_related()``API while the later uses Python
to perform in-memory JOINs.

class Article(models.Model):
    publisher = ForeignKey(Publisher, related_name='articles')
    medium = ForeignKey(Medium)
    billing = ForeignObject(
        'PublisherMediumBilling', from_fields=['publisher', 'medium'], to_fields=['publisher', 'medium']
    )

publisher.articles.select_related('billing')

Cheers,
Simon

Le samedi 18 juin 2016 07:09:03 UTC-4, Ram Jayaraman a écrit :

I apologize if this is an old question, I did search SO/google for a good few hours.

I have the following models.

Publisher()

  name = CharField()

Medium()

  name = CharField()

Article()

  publisher = ForeignKey(Publisher, related_name="articles")

  medium = ForeignKey(Medium, related_name="articles")

  text = TextField()

PublisherMediumBilling()

  publisher = ForeignKey(Publisher()

  medium = ForeignKey(Article)
  lots_of_other_columns

 ...
  class Meta:
      unique_together = ['publisher', 'medium']

I am looking to query all Article from a given publisher, and also fetch the related PublisherMediumBilling joining on the columns Publisher and Medium, Prefetch() object doesn't seem to quite work for this case.

I can of course filter-query for all PublisherMediumBilling and cache it and do a in-memory Python JOIN. Is there a way to do through one of the ORM Apis ?

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