[Django]-Many-To-One Relation Query in Django

One way:

group = Group.objects.get(pk=group_id)
contacts_in_group = Contact.objects.filter(group=group)

Another, more idomatic, way:

group = Group.objects.get(pk=group_id)
contacts_in_group = group.contact_set.all() 

contact_set is the default related_name for the relation as shown in the related objects docs.

If you wanted to, you could specify your own related_name, such as related_name='contacts' when defining the field and then you could do group.contacts.all()

To add a new Contact to a group, you just need to assign the relevant group to contact via the group field and save the Contact:

the_group = Group.objects.get(pk=the_group_id)
newcontact = Contact()
...fill in various details of your Contact here...
newcontact.group = the_group
newcontact.save() 

Sounds like you’d enjoy reading the free Django Book to get to grips with these fundamentals.