def add_lazy_relation(cls, field, relation, operation):
    # Check for recursive relations
    if relation == RECURSIVE_RELATIONSHIP_CONSTANT:
        app_label = cls._meta.app_label
        model_name = cls.__name__

    else:
        # Look for an "app.Model" relation

        if isinstance(relation, six.string_types):
            try:
                app_label, model_name = relation.split(".")
            except ValueError:
                # If we can't split, assume a model in current app
                app_label = cls._meta.app_label
                model_name = relation
        else:
            # it's actually a model class
            app_label = relation._meta.app_label
            model_name = relation._meta.object_name

    # Try to look up the related model, and if it's already loaded resolve the
    # string right away. If get_model returns None, it means that the related
    # model isn't loaded yet, so we need to pend the relation until the class
    # is prepared.
    model = get_model(app_label, model_name,
                      seed_cache=False, only_installed=False)
    if model:
        operation(field, model, cls)
    else:
        key = (app_label, model_name)
        value = (cls, field, operation)
        pending_lookups.setdefault(key, []).append(value)
Adds a lookup on ``cls`` when a related field is defined using a string,
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/models/fields/related.py/add_lazy_relation