test.rs

impl<T, U> ToJsonApi for Vec<(T, Option<U>)>
where T: ToResourceObject + Default, U: ToResourceObject
{
  fn serialize(&self) -> String {
    self.into_iter().group_by(|&&(a, b)| a)
      .map(|(c, d)| d.into_iter().filter_map(|(e,f)| f).collect())
      .collect();

  "some".to_string()
  }
}