Redmine/vendor/plugins/engines/generators/plugin_migration/templates/plugin_migration.erb

class <%= class_name %> < ActiveRecord::Migration
  def self.up
  <%- plugins.each do |plugin| -%>
    Engines.plugins["<%= plugin.name %>"].migrate(<%= new_versions[plugin.name] %>)
  <%- end -%>
  end

  def self.down
  <%- plugins.each do |plugin| -%>
    Engines.plugins["<%= plugin.name %>"].migrate(<%= current_versions[plugin.name] %>)
  <%- end -%>
  end
end