python - How to fix manytomany field in django

Question

How to make a one to many relationship in Django/Mysql?

I have an identical situation to this post, yet, my django returns errors on the admin page:

get() returned more than one order2pizza-- it returned 5!

order2pizza with that pizza already exists.

My mysql database have composite keys on a tertiary table to order and pizza to link multiple pizzas to an order.

models.py:

class Orders(models.Model):
    order_id = models.AutoField(primary_key=True)
    order_name = models.CharField(max_length=100, blank=True, null=True)


class Pizza(models.Model):
    Pizza= models.AutoField(primary_key=True)
    Pizza_name= models.CharField(max_length=50, blank=True, null=True)

class order2pizza(models.Model):
    order = models.ManyToManyField(Orders, models.DO_NOTHING, )
    pizza_id= models.IntegerField()
    class Meta:
        unique_together = (('order ', 'pizza_id'),)

question from:https://stackoverflow.com/questions/65909707/how-to-fix-manytomany-field-in-django

Answer 1

A many-to-many relation can be expressed in two ways. First, you can manually specify a "join" model, like this:

class Orders(models.Model):
    order_name = models.CharField(max_length=255, blank=True)

class Pizza(models.Model):
    Pizza_name= models.CharField(max_length=255, blank=True)

class Order2Pizza(models.Model):
    order = models.ForeignKey(Order, models.CASCADE)
    pizza = models.ForeignKey(Pizza, models.CASCADE)
    class Meta:
        unique_together = ['order ', 'pizza']

This is useful if you want to put extra fields on the Order2Pizza model. A field named quantity would be very useful in your example.

The second option is to use a ManyToManyField. This will automatically create the join model for you:

class Orders(models.Model):
    order_name = models.CharField(max_length=255, blank=True)
    pizzas = models.ManyToManyField('Pizza', related_name='orders')

class Pizza(models.Model):
    Pizza_name= models.CharField(max_length=255, blank=True)

In your original question you put the ManyToManyField on the Order2Pizza model, which is nonsensical.

However, the source of your bug is probably your manual inclusion of several *_id fields. Don't do that. They will always be created automatically by Django and you should never have to specify them manually. Instead, try the two options above and see how they work.