One source of difficulty for people who are used to relational databases - and certain ORMs in particular - is how to handle references and relationships on App Engine. There's two basic questions here: First, what does a relationship entail, in any database system? And second, how do we use them in App Engine?
The nature of relationships
Many ORMs expose multiple 'types' of relationships as first-class entities - one-to-one, one-to-many, and many-to-many. This obscures the fact that, in reality, they are all built on the same building block, that of references. A reference is simply a field of an entity that contains the key of another entity - for example, if a Pet references an Owner, that simply means the Pet has a field that contains the key of its owner.
All relationship types simply devolve to references. A one-to-many relationship is a reference in its simplest form - each Pet has one Owner, and therefore an Owner can have multiple pets, all pointing to it. The Owner is not modified; it relies on the individual Pets naming it as the owner. One-to-one relationships are one-to-many relationships with the additional constraint that there will be only one Pet referencing each Owner; this is ...