A database system and method allows a user to write program code in a high-level programming language that implements a class that defines the structure of a user-defined aggregate and methods that can be invoked on instances of the user-defined aggregate. The class is then registered with the database system, which enforces a specific contract for user-defined aggregates against the class. The contract comprises the following requirements. First, the class must specify one of a plurality of different formats for persisting instances of the user-defined aggregate in a database store. Second, the class must comprise certain specified methods that can be invoked on an instance of the class. Once these requirements are satisfied, the database system enables instances of the user-defined aggregate to be created. Preferably, the database system stores metadata about the user-defined aggregate for subsequent use in creating instances of the aggregate.