A database system comprising a first database including a database manager and a schema with set of rules enabling the database manager to store and access data items in the first database, and a second database storing a plurality of data items. The schema further including rules to enable the database manager to access at least a subset of the data items in the second database. The first database further includes a first link pointing from the primary item to a replica of the secondary item stored in the first database, and a second link pointing from the replica to the secondary item in the second database.