speed up migration to identities

db/migrate/20141121161704_add_identity_table.rb

@@ -8,9 +8,11 @@ class AddIdentityTable < ActiveRecord::Migration
 
     add_index :identities, :user_id
 
-    User.where("provider IS NOT NULL").find_each do |user|
-      execute "INSERT INTO identities(provider, extern_uid, user_id) VALUES('#{user.provider}', '#{user.extern_uid}', '#{user.id}')"
-    end
+    execute <<eos
+INSERT INTO identities (provider, extern_uid, user_id)
+SELECT provider, extern_uid, id FROM users
+WHERE provider IS NOT NULL
+eos
 
     remove_column :users, :extern_uid
     remove_column :users, :provider
@@ -20,11 +22,15 @@ class AddIdentityTable < ActiveRecord::Migration
     add_column :users, :extern_uid, :string
     add_column :users, :provider, :string
 
-    User.where("id IN(SELECT user_id FROM identities)").find_each do |user|
-      identity = user.identities.last
-      user.extern_uid = identity.extern_uid
-      user.provider = identity.provider
-      user.save
+    if ActiveRecord::Base.connection.adapter_name == 'PostgreSQL'
+      execute <<eos
+UPDATE users u
+SET provider = i.provider, extern_uid = i.extern_uid
+FROM identities i
+WHERE i.user_id = u.id
+eos
+      else
+        execute "UPDATE users u, identities i SET u.provider = i.provider, u.extern_uid = i.extern_uid WHERE u.id = i.user_id"
       end
 
     drop_table :identities