Better support for objects that don't have a primary key called :id

[camsaul]

I think we could add a method to IModel called primary-key or something like that that could be used to get field(s) that should be used as the primary key (default implementation would return :id). It could accept either a keyword (for a single-column key) or a sequence of keywords for multiple-column keys.

[deanmarc25]

@camsaul I found this project and decided to play with this issue. Was wondering if you could clarify your vision for having multiple column keys. I used a simple map for my test run but am willing to fit it into the interface you described above before I finish the rest of the code.

Right now you can override the id field and the existing API won't change or you can pass a hash map into invoke-model to select on multiple fields.

Thanks!

[camsaul]

Implemented by #53