後から親は替えられない。
http://code.google.com/appengine/docs/datastore/keysandentitygroups.html
Entity Groups, Ancestors and Paths
Every entity belongs to an entity group, a set of one or more entities that can be manipulated in a single transaction. Entity group relationships tell App Engine to store several entities in the same part of the distributed network. A transaction sets up datastore operations for an entity group, and all of the operations are applied as a group, or not at all if the transaction fails.
When the application creates an entity, it can assign another entity as the parent of the new entity. Assigning a parent to a new entity puts the new entity in the same entity group as the parent entity.
An entity without a parent is a root entity. An entity that is a parent for another entity can also have a parent. A chain of parent entities from an entity up to the root is the path for the entity, and members of the path are the entity's ancestors. The parent of an entity is defined when the entity is created, and cannot be changed later.
Every entity with a given root entity as an ancestor is in the same entity group. All entities in a group are stored in the same datastore node. A single transaction can modify multiple entities in a single group, or add new entities to the group by making the new entity's parent an existing entity in the group.
