Easy query for Ref type should be added

[yokolet]

For example, given the entity definition and instances below:

class Person
  include Diametric::Entity
  include Diametric::Persistence

  attribute :name, String, :cardinality => :one, :doc => "A person's name"
  attribute :spouse, Ref, :cardinality => :one, :doc => "A person's spouse"
end
Person.create_schema

alice = Person.new(:name => "Alice")
alice.save

bob = Person.new(:name => "Bob", :spouse => alice)
bob.save

alice.spouse= bob
alice.save

currently, there's no easy way to find entity by "a person whose spouse's name is Alice" since spouse method returns dbid.