Question

I have a broker model and a city model with the Broker model defined like this:

class Broker(models.Model):
    user = models.ForeignKey(Auth)
    areas_of_operation = models.ManyToManyField(City)
    phone = models.CharField(max_length=20)
    company_name =  models.CharField(max_length=50)
    address1 = models.CharField(max_length=100)
    address2 = models.CharField(max_length=100)
    city = models.ForeignKey(City)
    state = models.ForeignKey(State, unique=True)
    zip = models.IntegerField(max_length=5)

Of course this will yield an error but I was wondering if there was any way I can facilitate a relationship that indicates that a Broker can work in many cities but lives in only one city. Or is this a high level design issue in which I must create more tables to show that relationship?

Answer 1

When you create a relationship from one model to the other, Django automatically adds a reverse relationship in the other direction. For example, if you got rid of the areas_of_operation field temporarily, it would work and you could use the code city.broker_set() to get all the brokers who live in a given city.

However, when you create multiple links from one model to another, Django tries to create multiple reverse relationships under the same attribute name. This is borne out by the error message when you run manage.py validate on your models:

Error: One or more models did not validate:
myapp.broker: Accessor for field 'city' clashes with related m2m field 'City.broker_set'. Add a related_name argument to the definition for 'city'.
myapp.broker: Accessor for m2m field 'areas_of_operation' clashes with related field 'City.broker_set'. Add a related_name argument to the definition for 'areas_of_operation'.

In other words, the problem is not in having multiple links from Broker to City, but rather the automatic reverse relationship having the same default name for both links. To get around this, use the related_name argument to set the name to use for the reverse relationship. The following code worked for me:

from django.contrib.auth.models import User
from django.db import models

class State(models.Model):
    name = models.CharField(max_length=100)

class City(models.Model):
    name = models.CharField(max_length=100)

class Broker(models.Model):
    user = models.ForeignKey(User)
    areas_of_operation = models.ManyToManyField(City, related_name='operators')
    phone = models.CharField(max_length=20)
    company_name =  models.CharField(max_length=50)
    address1 = models.CharField(max_length=100)
    address2 = models.CharField(max_length=100)
    city = models.ForeignKey(City, related_name='citizens')
    state = models.ForeignKey(State, unique=True)
    zip = models.IntegerField(max_length=5)

Now, if you have a City object, you can call city.operators() to get a list of who operates in that city, and city.citizens() for a list of who lives there.

See the Django model field documentation for further information on relationship fields.