Fix a typo in get_instances that broke modifying fields

Also added a test case to cover this behavior.

The error was:

    TypeError: unsupported operand type(s) for +: 'builtin_function_or_method' and 'int'

And it's because model.id was being set to the builtin function "id" instead of the variable "n" from the enumerator.

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Given the nature of this bug, I suspect I'm the only idiot using this for something nontrivial. 

Also, I liked the old name of the project better.