Specifies a shard key with the field(s) specified.
@example Specify the shard key.
class Person
include Mongoid::Document
field :first_name, :type => String
field :last_name, :type => String
shard_key :first_name, :last_name
end
@since 2.0.0
def shard_key(*names)
self.shard_key_fields = names
end