[Multiple issue assignees] Remove id column from issue_assignees table

db/migrate/20170320171632_create_issue_assignees_table.rb

@@ -26,7 +26,7 @@ class CreateIssueAssigneesTable < ActiveRecord::Migration
   # disable_ddl_transaction!
 
   def up
-    create_table :issue_assignees do |t|
+    create_table :issue_assignees, id: false do |t|
       t.references :user, foreign_key: { on_delete: :cascade }, index: true, null: false
       t.references :issue, foreign_key: { on_delete: :cascade }, null: false
     end

db/schema.rb

@@ -495,7 +495,7 @@ ActiveRecord::Schema.define(version: 20170320173259) do
 
   add_index "index_statuses", ["project_id"], name: "index_index_statuses_on_project_id", unique: true, using: :btree
 
-  create_table "issue_assignees", force: :cascade do |t|
+  create_table "issue_assignees", id: false, force: :cascade do |t|
     t.integer "user_id", null: false
     t.integer "issue_id", null: false
   end