I'm trying to customize the cost report to be able grouping by a custom field (Supplier) on user entities.

I've been successful in having custom fields from users being listed among the group by options, but the report execution still displays none for each entry, which I believe is related to a missing join, but I'm not sure where should I put my hands. My best guess would be cost_query.rb but I thought it should have used the mixin to build the query....

Please also consider I'm totally new to both Ruby and Rails, meaning if you are willing to help, I need specific directions, not just hints.

I've attached the few files I've added/modified so far.

Obviously I'm willing to create a pull request of this feature if I get it to work....