data

@data={ "p4" => {
		'parent_id' => "p1"
		'id' => "p10"	
	},
"p2" => {
		'parent_id' =>"p1"
		'id'=>"p20"
	},
"p11" =>{
	'parent_id'=>"p20"
	'id' =>"p111"
	},
"p22" =>{
	'parent_id'=>"p2"
	'id'=>"p123"
	}
}

flatten

def child_nodes(root_node="p1")
   @data.select { |k,v| v.parent_id == root_node }
end

def flatten_childen(id,node_depth=1)
  if node_depth == 0
      #return something?
  else
     #decrease depth??
     child_nodes(id).merge(child_nodes(id).map { |a,b| b.id }.each { |a| child_nodes(a) })
  end
end