How to render custom field with to_xml?

Question

I am working on a custom accessor method like an example below:

class Forest < ActiveRecord : Base
  has_many :trees

  def total_water_usage
    # summarize each tree's water_usage in this forest.
    # return as string 
  end

end

class Tree < ActiveRecord : Base
  belongs_to :forest

end

That is, I need your help for 2 questions:

1. How can I access each tree just for an instance of Forest class. (Like example below, the total water usage shouldn't summarize another forest's tree)

   asiaForest = Forest.find_by_name( 'asia' )
   asiaForest.total_water_usage
   

2. How can I force this method to be rendered by to_xml method? for example, I think the result should be similar to this:

   asiaForest.to_xml
   <asiaForest>
      ...
      <total_water_usage>239000</total_water_usage>   
      ...
   </asiaForest>
   

Could you help me to do this?

Answer 1

To implement on a global model scale you could add this to your model file.

 alias_method :ar_to_xml, :to_xml

  def time_zone_offset
       get_my_time_zone_offset_or_something
  end

  def to_xml(options = {}, &block)
    default_options = { :methods => [ :time_zone_offset ]}
    self.ar_to_xml(options.merge(default_options), &block)
  end

Answer 2

The answers are as follows:

class Forest < ActiveRecord::Base
  has_many :trees

  def total_water_usage
    trees.sum(:water_usage)
  end

  def to_xml
    attributes["total_water_usage"] = total_water_usage
    attributes.to_xml({:root => self.class.element_name})
  end
end

class Tree < ActiveRecord::Base
  belongs_to :forest

  def water_usage
    # place your water usage calculation for a tree here
  end
end

Explanation: Part 1 of the question is answered in total_water_usage which will call water_usage in each trees and sum it.

Part 2 we have to override the to_xml method to include the total_water_usage key. Taken from the original to_xml method.

Answer 3

records.to_xml(:methods => :total_water_usage)