resolved #151

app/models/issue.rb

@@ -1,11 +1,15 @@
 class Issue < ActiveRecord::Base
+  before_save :set_sub_id
+
   # This is to avoid conflict with the :type attribute
   self.inheritance_column = nil
 
   belongs_to :user
   belongs_to :project
 
   validates_presence_of :title, :description, :user, :project, :type, :status
+  validates_presence_of :sub_id, unless: :set_sub_id
+  validates :sub_id, uniqueness: { scope: :project }
 
   # TODO: make a list of 5 most popular types of issues
   #       people can raise on design projects.
@@ -42,11 +46,13 @@ def show_url
     File.join(project.urlbase, 'issue', sub_id.to_s)
   end
 
-  def sub_id
-    (project.issue_ids.index(id) + 1).to_i
+  def self.find_from_project(project, sub_id)
+    project.issues.find_by_sub_id(sub_id)
   end
 
-  def self.find_from_project(project, sub_id)
-    Issue.find(project.issue_ids[sub_id.to_i - 1])
+  def set_sub_id
+    return if sub_id
+      sub_id = project.issues.count + 1
+      update_attribute(:sub_id, sub_id)
   end
 end

db/migrate/20150318141041_add_sub_id_to_issues.rb

@@ -0,0 +1,5 @@
+class AddSubIdToIssues < ActiveRecord::Migration
+  def change
+    add_column :issues, :sub_id, :integer
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20150312234604) do
+ActiveRecord::Schema.define(version: 20150318141041) do
 
   create_table "comments", force: true do |t|
     t.text     "body"
@@ -66,6 +66,7 @@
     t.integer  "type"
     t.datetime "created_at"
     t.datetime "updated_at"
+    t.integer  "sub_id"
   end
 
   create_table "notification_statuses", force: true do |t|