redmine/db/migrate/039_create_watchers.rb

class CreateWatchers < ActiveRecord::Migration[4.2]
  def self.up
    create_table :watchers do |t|
      t.column :watchable_type, :string, :default => "", :null => false
      t.column :watchable_id, :integer, :default => 0, :null => false
      t.column :user_id, :integer
    end
  end

  def self.down
    drop_table :watchers
  end
end