Query for a ManytoMany Field with Through in Django

Question

Hi All,

I have a models in Django that are something like this:

class Classification(models.Model):
  name = models.CharField(choices=class_choices)
  ...

class Activity(models.Model):
  name = models.CharField(max_length=300)
  fee = models.ManyToManyField(Classification, through='Fee')
  ...

class Fee(models.Model):
  activity = models.ForeignKey(Activity)
  class = models.ForeignKey(Classification)
  early_fee = models.IntegerField(decimal_places=2, max_digits=10)
  regular_fee = models.IntegerField(decimal_places=2, max_digits=10)

The idea being that there will be a set of fees associated with each Activity and Classification pair. Classification is like Student, Staff, etc.

I know that part works right.

Then in my application, I query for a set of Activities with:

activities = Activity.objects.filter(...)

Which returns a list of activities. I need to display in my template that list of Activities with their Fees. Something like this:

Activity Name
Student Early Price - $4
Student Regular Price - $5
Staff Early Price - $6
Staff Regular Price - $8

But I don't know of an easy way to get this info without a specific get query of the Fees object for each activity/class pair.

I hoped this would work:

activity.fee.all()

But that just returns the Classification Object. Is there a way to get the Fee Object Data for the Pair via the Activities I already queried?

Or am I doing this completely wrong?

Answer 1

First of all I think you named your field wrong. This:

fee = models.ManyToManyField(Classification, through='Fee')

should be rather that:

classifications = models.ManyToManyField(Classification, through='Fee')

as ManyToManyField refers to a list of related objects.

In general ManyToManyField, AFAIK, is only a django shortcut to enable easy fetching of all related objects (Classification in your case), with the association table being transparent to the model. What you want is the association table (Fee in your case) not being transparent.

So what I would do is to remove the ManyToManyField field from Activity and simply get all the fees related with the activity. And thenm if you need a Classification for each fee, get the Classification separately.

Answer 2

Considering michuk's tip to rename "fee" to "classification":

Default name for Fee objects on Activity model will be fee_set. So in order to get your prices, do this:

for a in Activity.objects.all():
    a.fee_set.all() #gets you all fees for activity

There's one thing though, as you can see you'll end up doing 1 SELECT on each activity object for fees, there are some apps that can help with that, for example, django-batch-select does only 2 queries in this case.