#262 Trees with Ancestry

Apr 18, 2011 | 9 minutes |

The Ancestry gem works like a tree but also allows us to fetch deeply nested messages in a single query. It provides a method to arrange them after fetching.

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          bash
        
bundle
rails g migration add_ancestry_to_messages ancestry:string
rake db:migrate

          Gemfile
        
gem 'ancestry'

          migration.rb
        
class AddAncestryToMessages < ActiveRecord::Migration
  def self.up
    add_column :messages, :ancestry, :string
    add_index :messages, :ancestry
  end

  def self.down
    remove_index :messages, :ancestry
    remove_column :messages, :ancestry
  end
end

          messages_controller.rb
        
def new
  @message = Message.new(:parent_id => params[:parent_id])
end

          messages_helper.rb
        
def nested_messages(messages)
  messages.map do |message, sub_messages|
    render(message) + content_tag(:div, nested_messages(sub_messages), :class => "nested_messages")
  end.join.html_safe
end

          messages/index.html.erb
        
<%= nested_messages @messages.arrange(:order => :created_at) %>

          messages/show.html.erb
        
<%= nested_messages @message.subtree.arrange(:order => :created_at) %>

          messages/_form.html.erb
        
<%= f.hidden_field :parent_id %>

          messages/_message.html.erb
        
<%= link_to message.content, message %>
...
<%= link_to "Reply", new_message_path(:parent_id => message) %> |

          application.css
        
.nested_messages {
  margin-left: 30px;
}

.nested_messages .nested_messages .nested_messages .nested_messages {
  margin-left: 0;
}