User available query builder + backend

Hi,

I look for a plugin/gem/else for a GUI+backend hooks solution to
filter contents in a RoR app.

Example:

Customer(:id, :name, :vat_number, :state)
Address(:id, :customer_id, :state, :city, :street, :phone)

User want to select Customers with Customer.state == USA &&
Address.city == NY. Pretty simple with named_scope, less if user want
to freely build-up the query.

Therefore, I look for a (user available) query builder & (simple)
reporting tool.

A traditional reporting tool (Crystal, …) seems to me too much for
requirements.

Hints?

Regards