# in mongoid.yml, set autocreate_indexes: true so that the index is created dynamically

$ bin/rails c
Loading development environment (Rails 3.0.0)
irb(main):001:0> class Person
irb(main):002:1>   include Mongoid::Document
irb(main):003:1>   field :name
irb(main):004:1>   field :email
irb(main):005:1>   index :name
irb(main):006:1> end
=> {:name=>{:unique=>false}}
irb(main):007:0> Person.create!(:name => "John", :email => "john@email.com")
=> #<Person _id: 4cb37d8229d5fa3003000001, name: "John", email: "john@email.com">
irb(main):008:0> p = Person.last
=> #<Person _id: 4cb37d8229d5fa3003000001, name: "John", email: "john@email.com">
irb(main):009:0> p.name = "Steve"
=> "Steve"
irb(main):010:0> p.save!
=> true
irb(main):011:0> p = Person.last
=> #<Person _id: 4cb37d8229d5fa3003000001, name: "John", email: "john@email.com">
irb(main):012:0> p.email = "new@email.com"
=> "new@email.com"
irb(main):013:0> p.save!
=> true
irb(main):014:0> p = Person.last
=> #<Person _id: 4cb37d8229d5fa3003000001, name: "John", email: "new@email.com">