ok i think this is very basic, but since I am new to Django I don't know how to handle this.
I need to copy an instance of a django-model. As explained here, there is a problem with copying ManyToMany relations. But the attachment "django-model-copying.diff" has that function I guess. So I don't know - does my Django already have that function? I don't know how to call it. Help would be appreciated.
The docs include directions on how to do this - including how to handle many to many relationships.
You can just do the following:
m = MyModel.objects.get(pk=1)
m.id = None
m.save()
That way new instance will be created with new id of course in case of any unique properties it will trigger errors during validation.
NOTE:
As for the function you've mentioned - it is not yet added to the trunk, the status is design decision needed, but if you know what you're doing you can manually apply the diff to your django instance - it's called patching btw. Here are some details about how to do it: http://ariejan.net/2007/07/03/how-to-create-and-apply-a-patch-with-subversion/.
__copy__()/__deepcopy__() functions to your model and then save the m2m values. Or maybe using pickle.. I'll try to solve this one out - quite interesting ;)
m.pk = None as well -- and doesn't fix m2m problem.I'll try to answer your actual problem, because what you are asking for in the question is a problem with your proposed solution, which as many have pointed out is not really ideal.
In the comments you mention this:
i'll try to explain.. So we have 2 models: User and Book. A User has a book called "Titanic" with some content. Now, another user wants a relation to that book too. But, the second user wants exactly the same book, but it should be called "Ship is going under".. I would copy the book, and rename it. - I know, i could also put the content of the book in another model - but my model is a little bit more complex.
Looks like you have three things to track:
For each Book, store its common information, for example:
class Author(models.Model):
name = models.CharField(max_length=50)
email = models.EmailField()
def __unicode__(self):
return unicode(self.name)
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.ManyToMany(Author)
isbn = models.CharField(max_length=13)
# and so on
def __unicode__(self):
return unicode(self.title)
Now, you need to store Book -> User relation, such as one user can have many books, two users may own the same book, with their own meta information attached.
class BookClub(models.Model):
username = models.ForeignKey(User)
book = models.ForeignKey(Book)
comments = models.TextField()
If you structure your models like that, you'll be able to do queries like:
It's worth noting that as of django 1.8 you may have UUIDFields. If you copy a model instance and then save it the unique constraint will fail on this column, in which case you have to do something like:
import uuid
m = MyModel.objects.get(pk=1)
m.pk = None
m.uuid = uuid.uuid4() //generate new uuid
m.save()
