code: @Nullable
      public Relationship getRelationship(@Nonnull String id) {
          if (id == null || id.trim().length() == 0) {
              throw new IllegalArgumentException("A relationship ID must be specified.");
          }
  
          return relationshipsById.get(id);
      }
docs: Gets the relationship with the specified ID.
  
  @param id the {@link Relationship#getId()} of the relationship
  @return the relationship in this model with the specified ID (or null if it doesn't exist).
  @see Relationship#getId()