class ActiveFedora::RDF::FieldMap::ResourceBuilder

symbol assigned to :using will used as the value.
unless :using has been set as an option to the index.as block on the property in question. In that case, the
Builds a FieldMap entry for a resource such as an ActiveFedora::Base object and returns the uri as the value

def find_values 

def find_values
  object.send(name).map(&index_field_config.term.fetch(:using, :uri))
end