I want to send an email to the responsible person (user_id) of a Task (project.task) when it changes to a closed stage. I created an automated action and a mail template. 
This mail is not necessary if the user who is changing the stage is the same who is going to receive the mail. 

I tried with the condition ["write_uid","!=","user_id"], so if write_uid (the last user that modified the record) is not equal to user_id (the responsible of this task) the action have to be performed. 

However, "user_id" does not contain what I expected (the id of the user), but the literal string "user_id".

Is it possible to compare 2 fields in the domains of an automated action?