You can use entity groups to model one root entity (e.g. customer) and multiple dependent child entities (e.g. order,cart items..). All entities belonging to one entity group (e.g. one single customer) can be retrieved together using an ancestor query - kind of like a 'join'. But unlike with SQL joins you cannot retrieve all entities of multiple entity groups in a single query. Also using entity groups like that gives you transactional consistency guarantees while reading and writing.
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