> class Hoge; include Mongoid::Document; field :name, type: String; validates :name, uniqueness: true; end
=> {:uniqueness=>true}
> h = Hoge.new(name: 'foobar')
=> #<Hoge _id: 51df9674d53e43ebb0000003, created_at: nil, updated_at: nil, deleted_at: nil, name: "foobar", sheet_id: nil, kase_id: nil>
> h.upsert
=> true
> h.new_record?
=> true
> h.name = 'piyo'
=> "piyo"
> h.upsert
=> false