def prefetch_related_objects(model_instances, *related_lookups):
    """
    Populate prefetched object caches for a list of model instances based on
    the lookups/Prefetch instances given.
    """
    if len(model_instances) == 0:
        return  # nothing to do

    related_lookups = normalize_prefetch_lookups(related_lookups)

    # We need to be able to dynamically add to the list of prefetch_related
    # lookups that we look up (see below).  So we need some book keeping to
    # ensure we don't do duplicate work.
    done_queries = {}    # dictionary of things like 'foo__bar': [results]

    auto_lookups = set()  # we add to this as we go through.
    followed_descriptors = set()  # recursion protection

    all_lookups = deque(related_lookups)
    while all_lookups:
        lookup = all_lookups.popleft()
        if lookup.prefetch_to in done_queries:
            if lookup.queryset:
                raise ValueError("'%s' lookup was already seen with a different queryset. "
                                 "You may need to adjust the ordering of your lookups." % lookup.prefetch_to)

            continue

        # Top level, the list of objects to decorate is the result cache
        # from the primary QuerySet. It won't be for deeper levels.
        obj_list = model_instances

        through_attrs = lookup.prefetch_through.split(LOOKUP_SEP)
        for level, through_attr in enumerate(through_attrs):
            # Prepare main instances
            if len(obj_list) == 0:
                break

            prefetch_to = lookup.get_current_prefetch_to(level)
            if prefetch_to in done_queries:
                # Skip any prefetching, and any object preparation
                obj_list = done_queries[prefetch_to]
                continue

            # Prepare objects:
            good_objects = True
            for obj in obj_list:
                # Since prefetching can re-use instances, it is possible to have
                # the same instance multiple times in obj_list, so obj might
                # already be prepared.
                if not hasattr(obj, '_prefetched_objects_cache'):
                    try:
                        obj._prefetched_objects_cache = {}
                    except (AttributeError, __HOLE__):
                        # Must be an immutable object from
                        # values_list(flat=True), for example (TypeError) or
                        # a QuerySet subclass that isn't returning Model
                        # instances (AttributeError), either in Django or a 3rd
                        # party. prefetch_related() doesn't make sense, so quit.
                        good_objects = False
                        break
            if not good_objects:
                break

            # Descend down tree

            # We assume that objects retrieved are homogeneous (which is the premise
            # of prefetch_related), so what applies to first object applies to all.
            first_obj = obj_list[0]
            prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(first_obj, through_attr)

            if not attr_found:
                raise AttributeError("Cannot find '%s' on %s object, '%s' is an invalid "
                                     "parameter to prefetch_related()" %
                                     (through_attr, first_obj.__class__.__name__, lookup.prefetch_through))

            if level == len(through_attrs) - 1 and prefetcher is None:
                # Last one, this *must* resolve to something that supports
                # prefetching, otherwise there is no point adding it and the
                # developer asking for it has made a mistake.
                raise ValueError("'%s' does not resolve to an item that supports "
                                 "prefetching - this is an invalid parameter to "
                                 "prefetch_related()." % lookup.prefetch_through)

            if prefetcher is not None and not is_fetched:
                obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)
                # We need to ensure we don't keep adding lookups from the
                # same relationships to stop infinite recursion. So, if we
                # are already on an automatically added lookup, don't add
                # the new lookups from relationships we've seen already.
                if not (lookup in auto_lookups and descriptor in followed_descriptors):
                    done_queries[prefetch_to] = obj_list
                    new_lookups = normalize_prefetch_lookups(additional_lookups, prefetch_to)
                    auto_lookups.update(new_lookups)
                    all_lookups.extendleft(new_lookups)
                followed_descriptors.add(descriptor)
            else:
                # Either a singly related object that has already been fetched
                # (e.g. via select_related), or hopefully some other property
                # that doesn't support prefetching but needs to be traversed.

                # We replace the current list of parent objects with the list
                # of related objects, filtering out empty or missing values so
                # that we can continue with nullable or reverse relations.
                new_obj_list = []
                for obj in obj_list:
                    try:
                        new_obj = getattr(obj, through_attr)
                    except exceptions.ObjectDoesNotExist:
                        continue
                    if new_obj is None:
                        continue
                    # We special-case `list` rather than something more generic
                    # like `Iterable` because we don't want to accidentally match
                    # user models that define __iter__.
                    if isinstance(new_obj, list):
                        new_obj_list.extend(new_obj)
                    else:
                        new_obj_list.append(new_obj)
                obj_list = new_obj_list
dataset/ETHPy150Open django/django/django/db/models/query.py/prefetch_related_objects