There are the following limitations in the current release of the Relational DAS:
No support for nulls. There is no support for SQL NULL type. It is not legal to assign PHP NULL to a data object property and the Relational DAS will not write that back as a NULL to the database. If nulls are found in the database on a query, the property will remain unset.
Only two types of SDO relationship. The metadata described below allows the Relational DAS to model just two types of SDO relationship: multi-valued containment relationships and single-valued non-containment references. In SDO, whether a property describes a single- or multi-valued relationship, and whether it is containment or non-containment, are independent. The full range of possibilities that SDO allows cannot all be defined. There may be relationships that it would be useful to model but which the current implementation cannot manage. One example is a single-valued containment relationship.
No support for the full range of SDO data types. The Relational DAS defines all primitive properties in the SDO model as being of type string. SDO defines a richer set of types containing various integer, float, boolean and data and time types. String is adequate for the purposes of the Relational DAS since the combination of PHP, PDO and the database will ensure that values passed as strings will be converted to the proper type before being put in the database. This does affect some scenarios in which the Relational DAS has to work with a data graph that has come from or will go to a different DAS.
Only one foreign key per table. The metadata only provides the means to specify one foreign key per table. This foreign key may be mapped to one of the two types of SDO relationship supported. Obviously there are some scenarios that cannot be described under this limitation - it is not possible to have two non-containment references from one table to another for example.