Question

I am confused as far as Database design goes for a Django application with specific relationships. I have been looking at the following database schema : http://www.databaseanswers.org/data%5Fmodels/customers_and_orders/index.htm

I am confused about the relationships between Customer_addresses and Customers, and Addresses.

I know that:

• one customer can have Many addresses.
• many customers can have the same (one) address

Does this equate to a many to many relationship?

• Many customers can have many addresses?

When I’m building the model in django I have (simplified):

class Customer_Address(models.Model):
    customer = models.ManyToManyField('inventory.Customer')
    address = models.ManyToManyField('inventory.Address')

Is this right? or does this make more sense:

class Customer_Address(models.Model):
    customer = models.ForeignKeyField('inventory.Customer')
    address = models.ManyToManyField('inventory.Address')

UPDATED QUESTION:

Based on the idea that it is a many to many relationship.
Does one ManyToMany relation, require two ManyToManyFields?

Based on the following:
http://en.wikipedia.org/wiki/Many-to-many_%28data_model%29

Since Django supports ManyToManyFields, do I not even require the junction table?
And if I do use the junction table (Customer_addresses) the article seems to imply using Two OneToMany relationships, which wouldn’t that just be two foreign keys like this?

class Customer_Address(models.Model):
    customer = models.ForeignKeyField('inventory.Customer')
    address = models.ForgeinKeyField('inventory.Address')

UPDATED QUESTION:

Now that we know that Django builds the intermediate table, and the table Customer_Addresses is not required. Which table should have the ManyToManyfield? the customer tables? or the Address table?

Example: https://docs.djangoproject.com/en/dev/topics/db/models/#intermediary-manytomany

In the Person/Groups examples.. The Groups has the ManyToManyField. Is there any reason the ManyToManyfield couldn’t be defined in the Person table?

Answer 1

To Sum up

For the first question.

When a relationship states that

• one customer can have Many addresses.

• many customers can have the same (one) address

Than its clearly a Many-to-Many relationship and always use ManyToManyField

Now for the second question.

What if you want to have a intermediate table which has extra fields between two many to many entities(Tables/models)

Than create a new Model which has a ForiegnKey from the other two models. As in

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __unicode__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

For more Read