group-example.rb

require 'mongo_mapper'
require 'yaml'

MongoMapper.database = 'tmp'

class Foo
  include MongoMapper::Document
  key :name, String
end

Foo.collection.remove

8.times { Foo.create(:name => 'foo') }
4.times { Foo.create(:name => '') }
2.times { Foo.create() }

puts "Total       : #{MongoMapper.database.eval('db.foos.count()')}"
puts "name = foo  : #{MongoMapper.database.eval('db.foos.count({name: \'foo\'})')}"
puts "name = ''   : #{MongoMapper.database.eval('db.foos.count({name: \'\'})')}"
puts "name = null : #{MongoMapper.database.eval('db.foos.count({name: null})')}"
# puts "name = nil: #{MongoMapper.database.eval('db.foos.count({name: {$ne: \'foo\'}})')}"
# puts MongoMapper.database.eval('db.foos.count({name: {$ne: \'foo\'}})')

puts '--'

puts Foo.collection.group(['name'], {'name' => {'$ne' => {'$nin' => [nil,'']}}}, {'sum' => 0}, "function(doc, prev) { prev.sum += 1}", nil).to_yaml