Re: How to make a single 'special' m2m relationship?

I'm fairly new to Django but I worked through a similar problem a
couple months ago. You say:

>If I was doing this manually with SQL, I would probably make a table
>especially for the m2m relationship, called "articles_tags", and this
>table would have ariticle_ids linked to tag_ids, and then I'd add a
>boolean field to this table called something like "is_primary".

You can do that with Django models in a completely straighforward way:

class Tag(models.Model):
name = models.CharField(max_length=30,db_index=True)
def __unicode__(self):
return self.name

class Article(models.Model):
title = models.CharField(max_length=40,unique=True,db_index=True)
def __unicode__(self):
return self.title

class ArticleAndTag(models.Model):
class Meta:
unique_together = ('tag','article')
tag = models.ForeignKey('Tag',related_name='articleAndTagsForTag')
article =
models.ForeignKey('Article',related_name='articleAndTagsForArticle')
isPrimary = models.BooleanField(default=False)
def __unicode__(self):
return u'A %s T %s' %
(self.article.__unicode__(),self.tag.__unicode__())

If you've looked at ManyToManyFields with optional through=table
parameter, this is
basically what you would define for the through table – only you just
use it directly
and don't bother defining the m2m field in either your article or your
tag models.
All the m2m field really gives you is another pair of managers/
querysets (one defined
by you, one added by Django) which IMO clutter your models and don't
add much
incremental value beyond the ones you get directly from the table
definition.
The one thing you have to do manually when you don't have an m2m
field is extract
the fields you want from the pair objects, as:

print '\narticles\n'
for article in Article.objects.all():
print article
for articleAndTag in article.articleAndTagsForArticle.all():
print '\t',articleAndTag.tag,articleAndTag.isPrimary

print '\ntags\n'
for tag in Tag.objects.all():
print tag
for articleAndTag in tag.articleAndTagsForTag.all():
print '\t',articleAndTag.article,articleAndTag.isPrimary

-- John

On Jan 9, 1:23 pm, callum <callum.lo...@gmail.com> wrote:
> Hello, I'm new to Django (and to MVC, in fact) but I'm liking it a
> lot.
>
> I followed the beginners tutorial, and I'm now trying to put together
> a very simple "articles" app, which has these models so far:
>
> class Tag(models.Model):
>     name = models.CharField(max_length=30)
>     def __unicode__(self):
>         return self.name
>
> class Article(models.Model):
>     title = models.CharField('Article title', max_length=40)
>     pub_date = models.DateTimeField('Publish date')
>     ready_to_publish = models.BooleanField()
>     tags = models.ManyToManyField(Tag)
>     def __unicode__(self):
>         return self.short_title
>
> Here's my question... I want articles to have multiple tags, which is
> why I've used "models.ManyToManyField". But when I design the HTML
> page, I want one 'primary' tag to appear at the top of the article
> post, by the headline, and then the rest of the tags to appear at the
> bottom. So I need a way of specifying a 'primary tag' for each
> article. In other words, I might want to give the article the tags
> "art", "color", and "painting", but I want to specify that "painting"
> is the primary tag for this article.
>
> I tried adding the following property to the Article class:
>
>     primary_tag = models.ForeignKey(Tag)
>
> ...but then when I run syncdb, I get the following error message:
>
> Error: One or more models did not validate:
> articles.article: Accessor for field 'primary_tag' clashes with
> related m2m field 'Tag.article_set'. Add a related_name argument to
> the definition for 'primary_tag'.
> articles.article: Accessor for m2m field 'tags' clashes with related
> field 'Tag.article_set'. Add a related_name argument to the definition
> for 'tags'.
>
> I don't really understand the error message. Am I approaching this the
> wrong way?
>
> If I was doing this manually with SQL, I would probably make a table
> especially for the m2m relationship, called "articles_tags", and this
> table would have ariticle_ids linked to tag_ids, and then I'd add a
> boolean field to this table called something like "is_primary".
>
> What is the correct way to approach this with Django?
>
> Thanks,
> Callum

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