Let's say you have a model named Book and a model named Category. Each book has one and only one category, denoted by a foreign key. Thus, you'll have the following models:
class Category(models.Model):
name = models.CharField(max_length=128)
class Book(models.Model):
name = models.CharField(max_length=128)
category = models.ForeignKey('Category')
Now, when you have a Book instance you can refer to its category using
the corresponding field. Furthermore, if you have a category instance,
by default, django adds an attribute to it named book_set which returns
a queryset with all the books that have this specific category.
So you can do something like:
category = Category.objects.get(pk=1)
print "Books in category {0}".format(category.name)
for book in category.book_set.all():
print book.name
Now, book_set is an attribute that django constructed for us and gave it this
name by default. Using the related_name attribute of foreign key you can give
this attribute whatever name you want (for example if I had definited category
as this
category = models.ForeignKey('Category', related_name='book_collection') then instead of category.book_set.all() I'd use category.book_collection.all()).
In any case, you rarely need to change the related_name, if at all in usual case
(I don't recommend it because it's easy to remember the django default x_set).
However there's a use case where it is required: When you have multiple
foreign keys from a model to another.
In this case there would be a clash (since django would try to create two x_set attributes to the same model) and you need to help by naming
the x_set attributes yourself.
For example, if my Book model was like this (had a category and a subcategory):
class Book(models.Model):
name = models.CharField(max_length=128)
category = models.ForeignKey('Category')
sub_category = models.ForeignKey('Category')
then the model would not validate unless you give one (or both) of the
ForeignKeys a related_name attribute so that the clash will be resolved.
For example you could do something like this:
class Book(models.Model):
name = models.CharField(max_length=128)
category = models.ForeignKey('Category', related_name='book_category_set')
sub_category = models.ForeignKey('Category', related_name='book_sub_category_set')