How to fix "the model is used as in intermediate model but it does not have foreign key to a model"?

Question

Error Message:

blogs.Permission: (fields.E336) The model is used as an intermediate model by 'blogs.Category.permission', but it does not have a foreign key to 'Category' or 'Permission'.

I have tried to add a Foreign Key to ‘Category’ under Permission model, same error still occurs.

models.py:

from django.db import models

class Category(models.Model):
    name = models.CharField(max_length=50)
    permission = models.ManyToManyField('Permission',
                                        related_name='category_permissions',
                                        through='Permission'
                                        )

    def __str__(self):
        return self.name


class Permission(models.Model):
    HIGH = 'High'
    MEDIUM = 'Medium'
    LOW = 'Low'
    CLASSIFICATION_CHOICES = [
        (HIGH, 'High'),
        (MEDIUM, 'Medium'),
        (LOW, 'Low')
    ]
    category_name = models.ForeignKey(Category, on_delete=models.CASCADE, related_name='category_name')
    name = models.CharField(max_length=100)
    description = models.TextField()
    platform = models.CharField(
        max_length=10,
        choices=PLATFORM_CHOICES,
        default=BOTH,
    )
    classification = models.CharField(
        max_length=10,
        choices=CLASSIFICATION_CHOICES,
        default=LOW,
    )

    def __str__(self):
        return self.name


class MobileApp(models.Model):
    name = models.CharField(max_length=200)
    icon = models.ImageField(upload_to='app_icons', blank=True, null=True)
    platform = models.CharField(
        max_length=10,
        choices=PLATFORM_CHOICES,
        default=IOS,
    )
    category = models.ManyToManyField('Category')
    provider = models.CharField(max_length=200)
    identifier = models.CharField(max_length=200)
    permission = models.ManyToManyField(Permission,
                                        related_name='mobile_app_permission',
                                        )

    def __str__(self):
        return self.name

I am trying to use ‘through’ argument to include the description field of the permission m2m for MobileApp and Category

Asked By: Jerry

||

Source

Answer 1

This is all kinds of broken.

Apparently you want your many-to-many to be between Category and MobileApp, with the through model as Permission. So the m2m field needs to declare that:

class Category(models.Model):
    name = models.CharField(max_length=50)
    permission = models.ManyToManyField('MobileApp',
                                        related_name='category_permissions',
                                        through='Permission'
                                        )

Secondly, as the error states, in the through model you need foreign keys to both sides. Plus, you need to give them sensible names and related_names. So:

class Permission(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE, related_name='permissions')
    mobile_app = models.ForeignKey('MobileApp', on_delete=models.CASCADE, related_name='permissions')

Finally, you don’t need to also define m2ms in the other direction to either Permission or Category. Remove the category and permission fields from MobileApp.

Answered By: Daniel Roseman

Answer 2

I had the same prolem but i forgot that the ManyToManyField needs to use two different models in the class parameters "class=" and "through=" look at my code:

User is another class model which is declared somewhere else.

class Teams(models.Model):
    owner = models.ForeignKey(User, related_name="teams_owner", on_delete=models.CASCADE, primary_key=True)
    operators = models.ManyToManyField(User, related_name="teams_opearators", through="Operator")    
    
class Operator(models.Model):
    team = models.ForeignKey(Teams, related_name="team", on_delete=models.CASCADE)
    operator = models.ForeignKey(User, on_delete=models.CASCADE, related_name="operator")
    active = models.BooleanField(default=False)

it’s wrong to use the code like this:

class Teams(models.Model):
    owner = models.ForeignKey(User, related_name="teams_owner", on_delete=models.CASCADE, primary_key=True)
    operators = models.ManyToManyField(Operator, related_name="teams_opearators", through="Operator")

Answered By: amirh_far

How to fix "the model is used as in intermediate model but it does not have foreign key to a model"?

Question:

Answers: