Rails "per-user" mutex

Question

Is there a way to configure a mutex in Rails that only prevents the same user from running that code concurrently (as opposed to preventing all concurrency in that section of code)?

For example, I have the following method:

  def self.update(user, day, segment, category_id)
    entry = find(user, day, segment)
    
    if entry.nil?
      entry = Entry.new(user: user, date: day, segment: segment)
    end

    entry[key_for_id(category_id)] = category_id
    entry.save!
  end

The problem is that if the same user somehow executes this method concurrently, then two separate Entry object will be created (as opposed to the first execution creating an Entry and the second updating it)

Creating a basic Mutex will solve the problem; but, I don't need to prohibit all concurrent executions of the critical section (which I assume can have significant performance implications), I just need to make sure that the same user can't enter the critical section concurrently (which is relatively rare; but has happened).

Is there a way to do this in Rails?

Answer 1

As suggested by @dbugger in the comments and this other post: Are ActiveRecord's find_or_create* methods fundamentally flawed?,

The solution was to add a unique index and a retry.

class AddUniqueIndexToEntries < ActiveRecord::Migration[7.0]
  def change
    add_index :entries, [:user_id, :date, :segment], unique: true
  end
end


    begin 
      entry = find(user, day, time)
      if entry.nil?
        entry = Entry.new(user: user, date: day, segment: segment)
      end
    
      entry[key_for_id(category_id)] = category_id
      entry.save!
    rescue ActiveRecord::RecordNotUnique
      retry
    end