Rails how to restrict attribute updates

I got an Object, Rating, with 2 fields, user_id and value.

class CreateRatings < ActiveRecord::Migration[5.1]
     def change
       create_table :ratings do |t|
          t.belongs_to :user
          t.integer :user_id
          t.decimal :value
       end
     end
end

On create i want to set user_id and value to the given values from the Controller:

@rating = Rating.create(user_id: 1, value: 2)

But after i created it, it should not be possible to change the user_id attribute. Just the value attribute. So after that:

@rating.update(user_id: 2, value: 3)

@rating.user_id should still return 1, but value should be 3.

My idea was to use before_update to revert the changes, but that does not look right to me.

Is the another way to do it?

i hope i could make it more clear whats my problem..

Thanks

Update

The Controller looks like this:

  def create
     Rating.create(rating_params)
  end

   def edit
     Rating.find(params[:id]).update(rating_params)
   end

   private

   def rating_params
      params.require(:rating).permit(:user_id, :value)
   end

You could do that with some strong_params. Simply don't allow user_id when updating. Something along these lines:

class RatingsController
  def create
    @rating = Rating.create(create_rating_params)
    ...
  end

  def update
    @rating = Rating.find(params[:id])
    @rating.update_attributes(update_rating_params)
    ...
  end

  private

  def create_rating_params
    params.require(:rating).permit(:user_id, :value)
  end

  def update_rating_params
    params.require(:rating).permit(:value)
  end
end