get value from manytomany field in django model?

Question

I Have to tables

class Book(models.Model):
    title = models.CharField(max_length=200)        
    authors = models.ManyToManyField(Individual, related_name="author_for", blank=True, null=True)
    illustrators = models.ManyToManyField(Individual, related_name="illustrator_for", blank=True, null=True)

class Unitary_Sale(models.Model):        
            book = models.ForeignKey(Book)
            quantity = models.IntegerField()
            unit_price = models.DecimalField(max_digits=15, decimal_places=3)
            sale = models.ForeignKey(Sale)

How can report book has been sold by author or illustrator?

by_author = {}
   for unit_sale in Unitary_sale.objects.all():
        author = unit_sale.book.authors
        by_authors[author] =  (unit_sale.quantity, unit_sale.quantity *unit_sale.unit_price)


Author   Qty  Amount($)
A        2     20
A&B      3     30

***one book has many author

Answer 1

authors is many-to-many, so you'll need to nest another loop. The author object you made like a list, eg:

for unit_sale in Unitary_sale.objects.all():
    for x in author:
       by_authors[x] = ....

Edit: actually, I noticed a mistake in how you are creating author. It should be:

author = unit_sale.book.authors.all()

Then you can use a for loop to iterate through all the Author objects as above.

Answer 2

Just be mindful about the number of db queries that are executed under the hood. I was hypnotized by the easy way to access and iterate over db relations in my code which resulted in ~900 db queries per one page.