Map
 has_and_belongs_to_many :layers

 def update_layer
    self.layers.each do |layer|
      #layer.readonly? # true
      layer.update_layer
    end unless self.layers.empty?
  end

SELECT * FROM `layers` INNER JOIN `layers_maps` ON `layers`.id = `layers_maps`.layer_id WHERE (`layers_maps`.map_id = 91 )

Layer
 has_and_belongs_to_many :maps

 def update_layer
   get_bounds
 end

  def get_bounds
    extent = "1,2,1,0"
    update_attribute(:bbox, extent)
    extent
  end

gives ActiveRecord::ReadOnlyRecord at update_attribute