Question

I have a Link and a Bookmark model like this:

class Link(models.Model):
    url = models.URLField(unique=True)        
    def __unicode__(self):
        return self.url

class Bookmark(models.Model):
    title=models.CharField(max_length=200)
    user=models.ForeignKey(User)
    link=models.ForeignKey(Link)        
    def __unicode__(self):
        return u'%s, %s' % (self.user.username, self.link.url)

Now within a view I see if a Link with a given url already exists.
This object is then passed next with the username to Bookmarks collection to see if a bookmark already exists with this username and Link instance already exists.

def bookmark_save_page(request):
    if request.method == 'POST':
        form = BookmarkSaveForm(request.POST)
        if form.is_valid():
            # Create or get Link
            link, dummy = Link.objects.get_or_create(url=form.cleaned_data['url'])
            # Create or get bookmark
            bookmark, created = Bookmark.objects.get_or_create(user=request.user, link=link)

            # Save bookmark to database
            bookmark.save()
            return HttpResponseRedirect('/user/%s/' % request.user.username)

This is the bit I don’t understand. How does it know how to take the url field inside Link model as a way of comparison? Is it because I had defined it in the Link model like this?

def __unicode__(self):
        return self.url

I am coming from .NET and there you have to define the GetHash() for the class as a way to specify how the instances should be compared against each other.

How does Python know this?
Thanks

Answer 1

I think you are asking “how does Django compare instances when filtering”, rather than “how does python compare objects”.

With the following line of code,

bookmark, created = Bookmark.objects.get_or_create(user=request.user, link=link)

Django is filtering on link object’s primary key. The __unicode__ method does not matter.

See the Django docs for comparing objects and queries over related objects for more info.