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Model Design Tips

The formal VO-DML standard describes in some detail tha various elements that make up the VO-DML metamodel. This documentation attempts to provide some practical guidance on how to use these elements especially in the context of the way that the VO-DML tooling interprets and makes choices in the expression of various serializations of the models.

ObjectType or DataType

  • ObjectTypes are expressed as a table in RDB cf DataTypes are typically columns in a table.
    • In XML and JSON the differences are not so obvious

  • ObjectTypes can be composed

    • This will result in foreign key references in RDBs, but in XML and JSON will usually result in child lists.

  • ObjectTypes have separate identity

    • Means that they need an identifying key in RDB.
      • can use the NaturalKey constraint to use an attribute as a key
    • Though they do not necessarily need an identifying key in XML and JSON unless they are referred to.

References

On of the key modelling tools in VO-DML is to be able to define a references to other parts of the model.

Contained References.

Natural Keys

The ability to set a NaturalKey constraint on an attribute is a new feature of VO-DML 1.1

<attribute>
    <vodml-id>Proposal.id</vodml-id>
    <name>id</name>
    <description>collection-specific identifier for the proposal</description>
    <datatype>
        <vodml-ref>ivoa:string</vodml-ref>
    </datatype>
    <multiplicity>
        <minOccurs>1</minOccurs>
        <maxOccurs>1</maxOccurs>
    </multiplicity>
    <constraint xsi:type="vo-dml:NaturalKey">
        <position>1</position>
    </constraint>
</attribute>
This can be used to indicate to the database serialization that the attribute should be used as the primary key rather than a surrogate primary key being generated.

The <position> element is intended to signify the position in a composite primary key.

Composite primary keys from the composition hierarchy

If the <position> element of the NaturalKey is set to 0 then that is used to indicate that a composite primary key should be formed from the NaturalKeys that are defined in the composition hierarchy (and it is expected that there is a NaturalKey in each element up that hierarchy) The implied extra NaturalKeys that make up the composite should not be explicitly signified in the VO-DML.

External references