Cannot subscribe to more than one field

Attempting to subscribe to more than one field will only return the first one in the list. There is some obvious offending code in executor.py:
`def subscribe_fields(exe_context, parent_type, source_value, fields):
exe_context = SubscriberExecutionContext(exe_context)

def on_error(error):
exe_context.report_error(error)

def map_result(data):
if exe_context.errors:
result = ExecutionResult(data=data, errors=exe_context.errors)
else:
result = ExecutionResult(data=data)
exe_context.reset()
return result

observables = []

# assert len(fields) == 1, "Can only subscribe one element at a time."

for response_name, field_asts in fields.items():
result = subscribe_field(exe_context, parent_type, source_value, field_asts, [response_name])
if result is Undefined:
continue

def catch_error(error):
exe_context.errors.append(error)
return Observable.just(None)

# Map observable results
observable = result.catch_exception(catch_error).map(
lambda data: map_result({response_name: data}))
return observable
observables.append(observable)

return Observable.merge(observables)
`

The line `return observable` causes a bunch of unreachable code and exits the loop early. However, commenting out that line does not appear to fix the problem - if it is removed, only the last item in the subscription list will return.

Comienza en executor.py, en subscribe_fields, y reproduce una suscripción que solicite varios campos. Rastrea cómo se convierte el resultado de cada campo en un Observable y cómo se devuelve el resultado combinado. Se considera terminado cuando todos los campos solicitados aparecen en el resultado de la suscripción y los errores siguen notificándose correctamente.

Escrito por el modelo de indexación a partir del texto del issue.