class Location < ApplicationRecord
  has_many :deals
  has_many :active_deals, -> { where(active: true) }, source: :deals
end

class LocationSerializer
  include FastJsonapi::ObjectSerializer
  has_many :deals, object_method_name: :active_deals
end

# create 1 deal for the location with active as false

pp LocationSerializer.new(@location).to_json

{"data"=>
  [{"id"=>"xxx",
    "type"=>"location",
    "attributes"=>
     {},
    "relationships"=>
     {"deals"=>{"data"=>[{"id"=>"xxx", "type"=>"deal"}]}}}],
 "included"=>[]}