A Case for a OneToMany Relationship in Django
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There are three types of model relationships that Django provides: many-to-one, many-to-many and one-to-one. In the following post I intend to make a case for adding a one-to-many relationship to this list.
Obviously, the immediate opposition to this proposal is that all relationships in Django are provided in a two-way manner (via
related_name), so there’s no need for it. Let’s take a look at two reasons why I think this new relationship is called for.
The many-to-one relationship that Django provides is
ForeignKey. A foreign key is always declared on the “one” side of the relationship, e.g., if you have a
Band model and each
Band has several
Musician models related to it, then you would put a
ForeignKey in the
Musician model like so:
This way, every musician has a band related to it, and bands may have musicians related to them, but this is the wrong way around! Musicians exist without bands. Some are in bands, some aren’t. Even those who are in a band aren’t defined by it. However, a band is determined by its members. A band without members cannot exist. So, there’s a semantic issue here. Therefore, I suggest the following syntax:
This way - every band has musicians. Musicians may be in bands, but not necessarily. And, and this is the difference between the proposed
OneToMany and the already-existing
ManyToMany, each musician is in only one band.
Let’s toss some more models into the mix. Suppose we have music agents and each one of them has musicians as clients. Perhaps we also have different musician unions. Using
ForeignKey again, it’ll look like this:
Band before), it’s confusing that a required piece of information about them is defined elsewhere, while in
Musician there are so many optional members that clutter up what’s important about the musician. Here’s how it’ll look like with
Every model contains all of its required information and at the same time they are not cluttered with a bunch of optional members. With
ForeignKey, the more “musician groups” we have, the more clutter is gathered in the
Musician model. So OneToMany helps keep the information where it has the most meaning to you, the programmer.
The previous example also suggests another advantage of the relationship. When working with an already-existing model, both
ManyToMany relationships allow us to add information to that model without altering its source code. However, when adding a many-to-one relationship, you must always place the ForeignKey in the “one” side, which may be a model which you can’t change, for any number of reasons (e.g., working with a library, or working with code that is common to several applications). The OneToMany relationship allows us to break that barrier.
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