Rails Active Record field, dynamic value (evaluation)

I'm trying to render a JSON object containing a list of Stores (Model: Store). Users have associations to Stores, however each User has a unique # of points available at each store for use. The expression to find the total count of points available for a store is :

@user.codes.unspent.find_all_by_store_id(@store.id).count
# unspent is a scope 
# @user is the currently logged in user
# Code model keeps all activity records of points. 

Because I am rendering a JSON object list of all the Stores, and the above expression is obviously not a field in the Store database, how can I evaluate this expression and return the count for each store?

Also, what are performance implic开发者_Python百科ations?

Thanks.

maybe this could help , from apidocs

  @events.to_json(:include => {
                    :images => {
                      :only => [], :methods => [:public_url] }})

it populate the json with custom method .