An integration server system for mapping data objects on a database schema offers a strongly-typed model API, complex constraint management, and association balancing. The integration server system also has improved performance through optimized handling of ordered associations of data object and of string-valued attributes. The integration server system comprises a database schema configured to store a set of data object instances. A metadata model represents a configuration of the set of data object instances in the database schema. A model application programming interface provides a client application with access to the set of data object instances, and a metadata application programming interface provides a client application with access to the set of data object instances via the metadata model.