Django ORM: Many to Many Self-Reference with Through Model - django

Question

I have a many-to-many self-referencing relationship that includes a through model. Products are composed of other Products, including an amount (I.E a screw is made from iron rods). I made the through model, although I can't seem to access the 'amount' field in the through model:
Model code:
from django.db import models
class Product(models.Model):
name = models.CharField(max_length=200, unique=True)
produced = models.IntegerField(default=0)
speed = models.IntegerField()
type = models.CharField(max_length=200)
ingredients = models.ManyToManyField('self', through='RecipeComponent', symmetrical=False)
def __str__(self):
return self.name
class RecipeComponent(models.Model):
item = models.ForeignKey(Product, on_delete=models.CASCADE, related_name="item")
ingredient = models.ForeignKey(Product, on_delete=models.CASCADE, related_name="ingredient")
amount = models.IntegerField()
def __str__(self):
return str(self.amount) + " of " + self.ingredient.name
I've tried a number of other queries, but as you can see they seem to fail. I know I'm missing something but I can't put my finger on it.
screw.ingredients.all()
>>> <QuerySet [<Product: iron_rod>]>
screw.ingredients.all()[0].amount
>>> Traceback (most recent call last):
>>> File "<input>", line 1, in <module>
>>> AttributeError: 'Product' object has no attribute 'amount'

Answer 1

screw.ingredients.all()[0]
Is a product, and if you try:
screw.ingredients.all()[0].amount
you're trying to access the amount attribute of Product which does not have. Hence the error:
>>> AttributeError: 'Product' object has no attribute 'amount'.
If you want to get the amount describing some relation you can:
relation_data = RecipeComponent.objects.get(product=screw, ingredient=wood)
relation_data.amount # The amount of wood for screw.
Or you want to know how many different ingredients has screw:
RecipeComponent.objects.filter(product=screw).count()

Answer 2

You can create a method within your Product class that returns the amount of the given ingredient:
class Product(models.Model):
...
...
def get_amount_of_ingredient(self, ingredient):
try:
return RecipeComponent.objects.get(item=self, ingredient=ingredient).amount
except RecipeComponent.DoesNotExist:
return 0
Then call it from screw object:
screw.get_amount_of_ingredient("iron_rod")