Add reviewer_id to review_of_review_mappings (metareview mappings)
user
```ruby
class AddReviewerIdToReviewOfReviewMappings < ActiveRecord::Migration
  def self.up
    begin
      add_column :review_of_review_mappings, :reviewer_id, :integer, :null => true
    rescue
    end

  end

  def self.down
  end
end
```
assistant