Django - How to retrieve data from intermediary model in many-to-many relationship?

I have the following Models set up in my app:

• Lesson
• Student
• Evaluation

The Lesson and Student models have a many-to-many relationships through Evaluation. In the Evaluation model, I also store the fee received for the lesson. I know I should really store this in the Lesson model and then multiply by the number of students but sometimes for whatever reason a student may not pay the full amount for a lesson, so I need to keep track of it on an individual basis.

I'm trying to create a list of all lessons in a given time period and include a column which displays

1. The total amount received for the lesson
2. The total amount received for all lessons in that time period.

At the moment, I am just using a generic view to return all lessons and I have a simple开发者_Python百科 method defined in my Lesson model which calculates the total fee received:

def total_fee(self):
    evaluations = self.evaluation_set.all()
    total = 0;
    for e in evaluations:
        total += e.fee_paid

    return total

However, I'm running the debug toolbar and this shows me that this view required 87 separate SQL queries when I ran it!

Obviously, I can't do that every time. How can I fetch all of the data in one go?

Any advice would be greatly appreciated.

Thanks.

I think I would personally tackle this one using Django's Aggregation functionality. It's hard for me to conceptualize your database structure without seeing your models, but I think the call would look something like this:

from django.db.models import Sum
total = self.evaluation_set.all().aggregate(total=Sum('fee_paid'))['total']

That should give you the sum of all the fees paid for everything in the evaluation set in 1 DB call, I think. Take that with a grain of salt as I haven't tested it.

Hope that helps.