All,
I am struggling to fix a database in such a way that I can make sure that
each entry is unique within its framework.
Suppose the following, in a library setting:
(Using Django 1.0 and SQL Server)
An author may have multiple books. The books may have the same price. They
may even have the same publisher. But each library branch only owns one
copy. (This is just an example case - so the analogy does not entirely work
in some cases, but you get the idea.)
So originally I created a table for with each book containing a separate
id. However, I realized that the same book could get entered twice, as I am
not checking for this scenario before I enter the newly received book. And
I cannot have duplicate entries. So I came up with the brilliant idea to
change the database to a composite key, consisting of the author, price,
publisher and library. I accordingly went in and changed the models.py -
something like this:
class Book(models.Model):
author_id = models.ForeignKey(Authors, db_column='Author_ID',
primary_key=True, editable=False)
price = models.DecimalField(decimal_places=2, db_column=u'Price')
publisher = models.ForeignKey(Publishers, db_column='Publisher')
library = models.ForeignKey(Libraries, db_column='Library')
class Meta:
unique_together = ('author_id', 'price', 'publisher', 'library')
Here is what views.py has:
def book_input(request, author_id):
a = get_object_or_404(Authors, pk=author_id)
if request.method == 'POST':
form = BookInputForm(author_id, request.POST)
if form.is_valid():
newbook = form.save(commit=False)
newbook.author_id = a
newbook.save()
return HttpResponseRedirect('/libraries/')
else:
form = BookInputForm(author_id)
return render_to_response('Book_Input.html', {'author':a, 'form':form,
})
Before I changed to this composite primary key idea, i could potentially get
a duplicate entry, but the entry did work. Now, when I try to enter any
item, the SQL server gives me an error saying:
"Violation of PRIMARY KEY constraint... Cannot insert duplicate key in
object...
Have I missed something, because I checked and the combination that I was
trying to insert is indeed unique based upon the criteria I have set in the
database?
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