fixes

app/models/index_status.rb

 class IndexStatus < ActiveRecord::Base
   belongs_to :project
 
-  validates :project_id, uniqueness: true
+  validates :project_id, uniqueness: true, presense: true
 end

db/migrate/20160129075828_create_index_statuses.rb

 class CreateIndexStatuses < ActiveRecord::Migration
   def change
     create_table :index_statuses do |t|
-      t.integer :project_id
+      t.integer :project_id, null: false
       t.datetime :indexed_at
       t.text :note
       t.string :last_commit
@@ -9,6 +9,6 @@ class CreateIndexStatuses < ActiveRecord::Migration
       t.timestamps null: false
     end
 
-    add_index :index_statuses, :project_id
+    add_index :index_statuses, :project_id, unique: true
   end
 end