Here's what it's doing:
* iterate over the fields
** if the field is content
*** put it in the local content dict if it's set locally (and not cached if .save() wasn't called)
*** if the content contains the xml data blob, assume it's the only content field and ignore all other fields by blitting the dict w/ the value of this one field. (so model_data is either a dict of field value pairs or just the string value of the data field)
*** rewrite the links in the 'data' part of the content
*** save it back to mongo