def prefetch_one_level(instances, prefetcher, lookup, level):
    """
    Helper function for prefetch_related_objects

    Runs prefetches on all instances using the prefetcher object,
    assigning results to relevant caches in instance.

    The prefetched objects are returned, along with any additional
    prefetches that must be done due to prefetch_related lookups
    found from default managers.
    """
    # prefetcher must have a method get_prefetch_queryset() which takes a list
    # of instances, and returns a tuple:

    # (queryset of instances of self.model that are related to passed in instances,
    #  callable that gets value to be matched for returned instances,
    #  callable that gets value to be matched for passed in instances,
    #  boolean that is True for singly related objects,
    #  cache name to assign to).

    # The 'values to be matched' must be hashable as they will be used
    # in a dictionary.

    rel_qs, rel_obj_attr, instance_attr, single, cache_name = (
        prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))
    # We have to handle the possibility that the QuerySet we just got back
    # contains some prefetch_related lookups. We don't want to trigger the
    # prefetch_related functionality by evaluating the query. Rather, we need
    # to merge in the prefetch_related lookups.
    # Copy the lookups in case it is a Prefetch object which could be reused
    # later (happens in nested prefetch_related).
    additional_lookups = [
        copy.copy(additional_lookup) for additional_lookup
        in getattr(rel_qs, '_prefetch_related_lookups', [])
    ]
    if additional_lookups:
        # Don't need to clone because the manager should have given us a fresh
        # instance, so we access an internal instead of using public interface
        # for performance reasons.
        rel_qs._prefetch_related_lookups = []

    all_related_objects = list(rel_qs)

    rel_obj_cache = {}
    for rel_obj in all_related_objects:
        rel_attr_val = rel_obj_attr(rel_obj)
        rel_obj_cache.setdefault(rel_attr_val, []).append(rel_obj)

    to_attr, as_attr = lookup.get_current_to_attr(level)
    # 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:
            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, model.__name__))

    # Whether or not we're prefetching the last part of the lookup.
    leaf = len(lookup.prefetch_through.split(LOOKUP_SEP)) - 1 == level

    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
            to_attr = to_attr if as_attr else cache_name
            setattr(obj, to_attr, val)
        else:
            if as_attr:
                setattr(obj, to_attr, vals)
                obj._prefetched_objects_cache[cache_name] = vals
            else:
                manager = getattr(obj, to_attr)
                if leaf and lookup.queryset is not None:
                    try:
                        apply_rel_filter = manager._apply_rel_filters
                    except __HOLE__:
                        warnings.warn(
                            "The `%s.%s` class must implement a `_apply_rel_filters()` "
                            "method that accepts a `QuerySet` as its single "
                            "argument and returns an appropriately filtered version "
                            "of it." % (manager.__class__.__module__, manager.__class__.__name__),
                            RemovedInDjango20Warning,
                        )
                        qs = manager.get_queryset()
                    else:
                        qs = apply_rel_filter(lookup.queryset)
                else:
                    qs = manager.get_queryset()
                qs._result_cache = vals
                # We don't want the individual qs doing prefetch_related now,
                # since we have merged this into the current work.
                qs._prefetch_done = True
                obj._prefetched_objects_cache[cache_name] = qs
    return all_related_objects, additional_lookups
dataset/ETHPy150Open django/django/django/db/models/query.py/prefetch_one_level