add uniqueness constraint on user names

db/migrate/20170414005856_add_unique_name_constraint_to_users.rb

@@ -0,0 +1,6 @@
+class AddUniqueNameConstraintToUsers < ActiveRecord::Migration
+  def up
+  	remove_index :users, :name
+  	execute "create unique index index_users_on_name on users(lower(name))"
+  end
+end