On Dec 11, 2007, at 8:02 PM, Bill Burke wrote: Consistency. Partially it has to do with avoiding circular references as you mention below, but that is really a choice for the person doing the mapping (hence why embed-xml is even an *option*). The bigger issue here imo is the fact that the nature of XML suggests that we are discussing structured data; structure which is generally described by a schema or DTD. What you do not like is the "static" nature of the choice... I am not "an XML person". However, I believe there is an approach which would be great and which XML supports, but I am not certain. I am talking about representing the association as a "link" (potentially) to another part of the document describing the associated data on the basis that it may be shared: <person id="1"> <name>Bill Burke</name> <!- here, an 'employer' element would "link" to the <company id="1"/> reference below --> </person> <company id="1"> ... </company> Then, if the association data is uninitialized you would still have the "link", but the entity data would not be included in the document. I think that would be a great feature/option and would gladly accept patches for that.