Add cards relation and format_id attribute to snapshot resource.

Author: plural

app/models/snapshot.rb

@@ -4,4 +4,5 @@ class Snapshot < ApplicationRecord
   belongs_to :format
   belongs_to :card_pool
   belongs_to :restriction, optional: :true
+  has_many :cards, through: :card_pool
 end

app/resources/api/v3/public/snapshot_resource.rb

@@ -4,14 +4,15 @@ module Public
       class Api::V3::Public::SnapshotResource < JSONAPI::Resource
         immutable
 
-        attributes :active, :date_start, :updated_at
+        attributes :format_id, :active, :date_start, :updated_at
         key_type :string
 
         paginator :none
 
         belongs_to :format
         has_one :card_pool
         has_one :restriction
+        has_many :cards
       end
     end
   end