How to order a set of objects based on association attributes in Ruby on Rails? -

in ruby on rails application, have model has number of one-to-one associations other models. let's each instance belongs region, country, , city. have selected couple of instance using scope , want order them based on name attributes of region, country, , city models not part of instance model itself.

the thing want show selected instances on index action of activeadmin page region, country, , city, , accordingly, want have them sorted in order.

it's straightforward order collection attributes on 1 or more associations—join on associations , order attributes. verbose way this:

instance.joins(:region, :country, :city).   order("regions.name desc").   order("countries.name desc").   order("cities.name desc") 

but recommend writing these conditions scopes on models. can change , reuse them more easily.

class instance < activerecord::base   scope :ordered, ->     joins(:region, :country, :city).     merge(region.ordered).     merge(country.ordered).     merge(city.ordered)   end end  class region < activerecord::base   scope :ordered, ->     order(name: :desc)   end end  class country < activerecord::base   scope :ordered, ->     order(name: :desc)   end end  class city < activerecord::base   scope :ordered, ->     order(name: :desc)   end end 

the magic here in merge method, documented here.

put like

@instances = instance.ordered