Error: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)

Question

I have the following action:

def index() {

  User.withNewTransaction {

   def user = User.get(params.userId)
   user.name = "test"
   user.save(flush:true)

   response.setContentType("image/gif")
   response.outputStream << PIXEL_BYTES_OF_A_GIF_IMAGE
   return
  }
}

When running, I sometimes get the following error:

Message
Executing action [index] of controller [test.TestController] caused exception: Runtime error executing action
Caused by
Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [test.User#1]

Why does this error happen? I thought withNewTransaction would prevent this error.

Solution

You can use pessimistic locking

Use:

User user = User.lock(params.userId)

or

User user = User.findById(params.userId, [lock: true])