ON CONFLICT DO NOTHING behaviour assumes existence of an 'id' field

[PeterJCLaw]

Thanks for this package, it looks like it could be very useful!

I've hit an issue with the DO NOTHING handling -- it assumes that a field called id exists on the model, though this isn't always the case.

class Foo(PostgresModel):
    user = models.OneToOneField(User, primary_key=True)

This results in a table with a primary key field called user_id,

It looks like the error stems from the way that the returning data is captured in the compilers' _rewrite_insert_nothing method which uses a literal id in the query construction.