[Django]-Many to many relation. ORM Django

You can use Django’s RelatedManager:

A “related manager” is a manager used in a one-to-many or many-to-many related context. This happens in two cases:

The “other side” of a ForeignKey relation. That is:

class Reporter(models.Model):
    ...

class Article(models.Model):
    reporter = models.ForeignKey(Reporter)

In the above example, the methods below will be available on the manager reporter.article_set.

Both sides of a ManyToManyField relation:

class Topping(models.Model):
    ...

class Pizza(models.Model):
    toppings = models.ManyToManyField(Topping)

In this example, the methods below will be available both on topping.pizza_set and on pizza.toppings.

These related managers have some extra methods:

1. To create a new object, saves it and puts it in the related object set. Returns the newly created object:
   create(**kwargs)

   >>> b = Toy.objects.get(id=1)
   >>> e = b.box_set.create(
   ...     name='Hi',
   ... )
   
   # No need to call e.save() at this point -- it's already been saved.
   
   # OR:
   
   >>> b = Toy.objects.get(id=1)
   >>> e = Box(
   ...     toy=b,
   ...     name='Hi',
   ... )
   >>> e.save(force_insert=True)
   

2. To add model objects to the related object set:

   add(obj1[, obj2, ...])
   

   Example:

   >>> t = Toy.objects.get(id=1)
   >>> b = Box.objects.get(id=234)
   >>> t.box_set.add(b) # Associates Box b with Toy t.
   

3. To removes the specified model objects from the related object set:

   remove(obj1[, obj2, ...])
   
   >>> b = Toy.objects.get(id=1)
   >>> e = Box.objects.get(id=234)
   >>> b.box_set.remove(e) # Disassociates Entry e from Blog b.
   

   In order to prevent database inconsistency, this method only exists on ForeignKey objects where null=True. If the related field can’t be set to None (NULL), then an object can’t be removed from a relation without being added to another. In the above example, removing e from b.entry_set() is equivalent to doing e.blog = None, and because the blog ForeignKey doesn’t have null=True, this is invalid.

4. Removes all objects from the related object set:
   clear()

   >>> b = Toy.objects.get(id=1)
   >>> b.box_set.clear()
   

   Note this doesn’t delete the related objects — it just disassociates them.
   Just like remove(), clear() is only available on ForeignKeys where null=True.

Reference: Relevant Django doc on handling related objects