def add_lazy_relation(cls, field, relation, operation):
    """
    Adds a lookup on ``cls`` when a related field is defined using a string,
    i.e.::

        class MyModel(Model):
            fk = ForeignKey("AnotherModel")

    This string can be:

        * RECURSIVE_RELATIONSHIP_CONSTANT (i.e. "self") to indicate a recursive
          relation.

        * The name of a model (i.e "AnotherModel") to indicate another model in
          the same app.

        * An app-label and model name (i.e. "someapp.AnotherModel") to indicate
          another model in a different app.

    If the other model hasn't yet been loaded -- almost a given if you're using
    lazy relationships -- then the relation won't be set up until the
    class_prepared signal fires at the end of model initialization.

    operation is the work that must be performed once the relation can be resolved.
    """
    # 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 __HOLE__:
                # 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)
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/Django-1.6.10/django/db/models/fields/related.py/add_lazy_relation