Before launching into creating a custom UDM, you must remember that the elements that you choose to add into a UDM are all coming from various Java objects on the server. The database schema that you provided when you created the site ensured the creation of a number of data access objects for all the tables defined in that schema, so you will see those objects and attributes in the wizards in the UDM Editor. For such objects, it is important to recognize that, as a designer, you will have the tendency to consider them as tables that you retrieve data from - and zeroCode reinforces that tendency somewhat, to reduce the complexity of referring to them as data access objects everywhere. In reality, though, they are objects and can be manipulated as such via custom UDMs.

Another important issue to remember is the fact that the UDM is a data model and is created in memory just before a page is delivered to the user. In terms of sequence, when a user hits a specific page, the UDM for that page is constructed in memory and then the data in that memory model is delivered to the page. There are no restrictions on the structure of that model, in the sense that you could choose to include multiple nodes, from different, unrelated tables, in the same UDM, if you so need them. Sibling nodes can be completely unrelated, but child nodes must be related in some manner in the schema. And child nodes can be made to restrict their data based on the retrieval of a parent node, since the memory model is constructed in the same sequence as the UDM’s structure.