Rails partial updates problem with hashes

问题

Rails ActiveRecord support partial updates and it works well most of the time, but in case we have serialized hash field AR executes update every time, even if nothing has been changed. Here example from rails console:

### Create class and table ###
>> class Xx < ActiveRecord::Base; serialize :params; end
=> Object 

>> ActiveRecord::Base.connection.execute "CREATE TABLE xxes(id serial, params text)"
SQL (43.8ms)  CREATE TABLE xxes(id serial, params text)
=> #<PGresult:0x112113380>


### Create record ###
>> r = Xx.create(:params => {:a => 1})
SQL (0.9ms)  INSERT INTO "xxes" ("params") VALUES ('--- 
:a: 1
') RETURNING "id"
=> #<Xx id: 1, params: {:a=>1}>


### Find this record ### 
>> x = Xx.find(1)
Xx Load (2.5ms)  SELECT "xxes".* FROM "xxes" WHERE "xxes"."id" = 1 LIMIT 1
=> #<Xx id: 1, params: {:a=>1}>


### Change nothing and call save ###
>> x.save
AREL (1.1ms)  UPDATE "xxes" SET "params" = '--- 
 :a: 1
' WHERE "xxes"."id" = 1
=> true 

Are there any workaround?

回答1:

x.save if x.changed?

http://ar.rubyonrails.org/classes/ActiveRecord/Dirty.html

来源：https://stackoverflow.com/questions/7448083/rails-partial-updates-problem-with-hashes