db_eval.rb

require 'pp'
require 'rubygems'
require 'mongo_mapper'
require 'benchmark'

MongoMapper.database = 'testing'

class Foo
  include MongoMapper::Document
  
  key :members, Array
end
Foo.collection.remove

Foo.create(:members => [{'John' => 1, 'Steve' => 4}])
Foo.create(:members => [{'Bill' => 4, 'Frank' => 2}])
Foo.create(:members => [{'John' => 3, 'Steve' => 1}])

puts Benchmark.measure {
  pp Foo.database.eval("
    function(value) {
      var results = [],
          docs = db.foos.find({}).forEach(function(doc) {
            if (doc.members) {
              doc.members.forEach(function(member) {
                for(key in member) {
                  if (key === value || member[key] === value) {
                    results.push(doc);
                  }
                }
              });
            }
          });
      return results;
    }
  ", 1)
}

# also try this
pp Foo.all('members.John' => {"$exists" => true})
# that works but is case sensitive
# and doesn't do values