Django OneToMany Relationship Clarification

Newcomers to Django might notice something strange about Django model relations. There is model.ForeignKey which represents many-to-one relationship, model.ManyToManyField which is the many-to-many relationship, but there is no one-to-many relationship!

Some argue that one-to-many relationship is redudant with many-to-one, and since it is not represented by an actual field in a relational database, it should not be implemented.

A quick google search even turns up full pages of results saying one-to-many is not possible.

http://stackoverflow.com/questions/6928692/how-to-express-a-one-to-many-relationship-in-django http://blog.amir.rachum.com/blog/2013/06/15/a-case-for-a-onetomany-relationship-in-django/ https://www.quora.com/What-is-the-best-way-to-implement-one-to-many-image-field-in-Django http://james.lin.net.nz/2014/09/17/django-should-have-onetomanyfield/

However, one-to-many is avaliable in Django ORM, as a query, not a model field.

class Transaction(models.Model):
	amount = models.IntegerField()
	date = models.DateTimeField(auto_now_add=True)

class TransactionItem(models.Model):
	transaction = models.ForeignKey('Transaction')
	amount = models.IntegerField()
	count = models.IntegerField()

Take for example, models Transaction and TransactionItem. It should be obvious that TransactionItem “belongs to” Transaction. It is possible to query for all TransactionItems belonging to one Transaction with:

transaction.transactionitem_set.all()

The query is the lowercase of the many-to-one model, followed by _set.

However, having to remember to make this extra query, while other frameworks such as Rails just use the one-to-many relationship is not ideal. Using a @property makes this a bit more conveinient.

class Transaction(models.Model):
	amount = models.IntegerField()
	date = models.DateTimeField(auto_now_add=True)
	@property
	def items(self):
		return self.transactionitems_set.all()

This makes querying TransactionItems as simple as calling transaction.items

Similarly, it may be conveinent to set up a setter with @items.setter def(selfundefined value) as well.