code: <T extends Entity> T getRelation(Class<T> valuesClass, String name,
              boolean cachable) {
          return instance.getRelation(this, valuesClass, name, cachable);
      }
docs: Get a relation by name for this entity. Ignore cache if cachable is
  false.
  
  @param valuesClass Class type of T.
  @param name Name of the relation attribute.
  @param cachable False if should not be cached.
  @return The related asset.