Re: Comparing ManyToManyFields

You'll first have to explain what you mean by 'reading from a file'
and 'comparing'.

Are we comparing against serialized model exports? Is the user
importing data from a csv file? What column(s) is(are) you using as
keys for matching db objects against the file?

Give us a little more details and we'll try giving some suggestions in return =)

Sincerely,
André Terra

On 6/28/11, Josh Lothian <lothian@ornl.gov> wrote:
> So I'm working on my first Django application, and I've run into a
> stumbling block. In simplest terms, I've got two objects:
>
> def Package(models.Model):
> name=models.CharField(max_length=255)
> version=models.CharField(max_length=128)
>
> def Machine(models.Model):
> hostname=models.CharField(max_length=255)
> installed_packages=models.ManyToManyField(Package)
>
> I'm reading in information from some file, and need to compare against
> objects already in the database. Ideally, I could just create these
> objects in my code, compare to the existing Django objects, and add a
> new entry if it's different from anything that exists already.
>
> However, I've got that dang ManyToManyField in there, meaning I need
> to first commit a Machine to the DB, and then add Packages to it.
> Otherwise, of course I get:
>
> ValueError: 'Machine' instance needs to have a primary key value before a
> many-to-many relationship can be used.
>
> This seems non-ideal, since there's a pretty good chance I'll just
> need to delete it from the database as a dupe of an existing Machine.
>
> I see a couple of ways around this:
>
> - Instantiate a Machine and a list of Packages, and compare them
> separately. This isn't ideal, as my real objects likely have more
> than 1 many-to-many relationship.
>
> - Do database queries and whatnot directly. Yuck!
>
> - Punt and just add/remove things from the database. This would
> probably work fine now, but later on could definitely lead to scaling
> issues.
>
> Am I missing an obvious solution here? I've scoured the docs and
> can't seem to come up with anything that doesn't make me feel icky.
>
> -josh
>
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