MongoDB tips

part1.rb

class Post 
  include Mongoid::Document
  field :author
  field :text
  field :publish_date, :type => Date
  embeds_many :comments
  index [:author, :asc]
end

class Comment
  include Mongoid::Document
  field :author
  field :text
  field :publish_date, :type => Date
  embedded_in :post, :inverse_of => :comments
  index [:publish_date, :desc]
end

part2.rb

class Post 
  include Mongoid::Document
  field :author
  field :text
  field :publish_date, :type => Date
  embeds_many :comments
  index [:author, :asc]
  index ["comments.publish_date", :desc]
end

class Comment
  include Mongoid::Document
  field :author
  field :text
  field :publish_date, :type => Date
  embedded_in :post, :inverse_of => :comments
end

part3.rb

m = 'function(){ this.tags.forEach( function(z){ emit( z , { count : 1 } ); } ); };'
r = <<-REDUCE
function( key , values ){
  var total = 0;
  for ( var i=0; i < values.length; i++ )
    total += values[i].count;
  return { count : total };
};
REDUCE
Post.collection.map_reduce(m, r)