Object Relationship Attribute Data Model for Semistructured Data
ORA-SS data model; ORA-SS schema diagram
When a database schema is designed, a data model is initially used to model the real world constraints that are taken into account in the design of the schema. For semi-structured database design, it is necessary to capture the following constraints: object classes, n-ary relationship types, attributes of object classes, attributes of relationship types, cardinality, participation and uniqueness constraints, ordering, irregular and heterogeneous structures, for both data- and document-centric data.
The ORA-SS (Object-Relationship-Attribute Data Model for Semi-structured Data) data model was designed  specifically to capture the constraints that are necessary for designing semi-structured databases, for normalization of schemas, and for defining views.