[1.8.x] Refs #25693 -- Avoided redundant calls to get_fields() in `to_attr` validation.

        rel_attr_val = rel_obj_attr(rel_obj)

        rel_obj_cache.setdefault(rel_attr_val, []).append(rel_obj)

    for obj in instances:

        instance_attr_val = instance_attr(obj)

        vals = rel_obj_cache.get(instance_attr_val, [])

    to_attr, as_attr = lookup.get_current_to_attr(level)

        # Check we are not shadowing a field on obj.

        if as_attr:

    # Make sure `to_attr` does not conflict with a field.

    if as_attr and instances:

        # We assume that objects retrieved are homogeneous (which is the premise

        # of prefetch_related), so what applies to first object applies to all.

        model = instances[0].__class__

        try:

                field = obj._meta.get_field(to_attr)

            model._meta.get_field(to_attr)

        except exceptions.FieldDoesNotExist:

            pass

        else:

            msg = 'to_attr={} conflicts with a field on the {} model.'

                raise ValueError(msg.format(to_attr, field.model.__name__))

            raise ValueError(msg.format(to_attr, model.__name__))

    for obj in instances:

        instance_attr_val = instance_attr(obj)

        vals = rel_obj_cache.get(instance_attr_val, [])

        if single:

            val = vals[0] if vals else None