Django: Many-to-Many Relationship with through= vs. separate Model

Question

I was just wondering why you need a Many-to-Many Relationship using through, if you just could use a separate Model?

For Example:

class Test(models.Model):
    name = models.CharField(max_length=100)
    description = models.TextField()

class Indicator(models.Model):
    name = models.CharField(max_length=100)
    target_value = models.IntegerField()

class TestResult(models.Model):
    test = models.ForeignKey(Test)
    indicator = models.ForeignKey(Indicator)
    actual_value = models.IntegerField()
    timestamp = models.DateTimeField(auto_now_add=True)

Why should I define a Many-to-Many Relationship? Thanks

Answer 1

You should add the ManyToMany attribute for three reasons:

First of all with this attribute you are explicitly declaring the many to many relstion thus correctly modeling your domain objects. This is very important because anybody seeing your models will understand the relations - if you don't have the ManyToMany attribute the reader will be confused on what you intended to do. After all, a central point in django philosophy is that "explicit is better than implicit"! In your case you should add an indicators = ManyToMany('Indicator', through='TestIndicator') attribute to the Test model.

Furthermore, the ManyToMany attribute will help you creating queries involving the related models without needing to use the intermediate object. For instance you could get all tests having a particular indicator name ( I know this can be done also through the ForeignKey but explicit is better than implicit)

Finally, adding the ManyToMany attribute to one of the models you can create inline admin forms to easily edit the relation. For instance in your case you'd add an inline in your TestAdmin through which you'd be able to add Indicators to this Test.