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When storing such an aggregate, metadata is stored as well. This includes standard metadata, such as when the aggregate has been created and who created it, but also custom metadata that the business engineer can define on the aggregate definition by using expressions. This custom metadata can be seen as identifying attributes for an aggregate, but are not necessarily unique. In the example above, the Person aggregate could have the following custom metadata attributes: FirstName, LastName, SocialSecurityNumber, CurrentJob, HouseholdSize. As you see, custom metadata for an aggregate can originate from different entities, or even be a complex expressions, for example NumberOfJobs is calculated as COUNT(COLLECT Job FROM Person.HasJobs).

Note
It is even possible to define custom metadata that does not originate from entities of the aggregate definition at all, since custom metadata consists of an expression. This is not advisable in general, as there is no guarantee that the information needed to fill this attribute is available when reading and updated an aggregate entry at a later point in time.

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