def compute_key(external_identifier)
      x = [external_identifier].flatten
      x.map! {|xx| xx.to_s}
      x.join("_")
    end
def compute_key(external_identifier)
      x = [external_identifier].flatten
      x.map! {|xx| xx.to_s}
      x.join("_")
    end
Generate a string key from an array of identifiers