RDF in Mozilla FAQ - Archive of obsolete content
use nsirdfcompositedatasource.
...because the nsirdfcompositedatasource interface is derived from nsirdfdatasource, it can be queried and modified just like an individual data source.
...the database property refers to an nsirdfcompositedatasource object, which contains the datasources from which the template is built.

A bird's-eye view of the Mozilla framework
using the database attribute of the xulelement, getlink() obtains an nsirdfcompositedatasource, which presents the individual datasources of the xulelement as a single rdf graph.
... var tocds = toctree.database; nsirdfcompositedatasource inherits from nsirdfdatasource.
... var resource = rdf.getresource(rdfid); nsirdfcompositedatasource inherits from nsirdfdatasource, which supports a gettarget() method for obtaining the nsirdfnode for a given resource and property.

nsIXULSortService
inherits from: nsisupports last changed in gecko 2.0 (firefox 4 / thunderbird 3.3 / seamonkey 2.1) method overview void insertcontainernode(in nsirdfcompositedatasource db, in nsrdfsortstate sortstateptr, in nsicontent root, in nsicontent trueparent, in nsicontent container, in nsicontent node, in boolean anotify); native code only!
...void insertcontainernode( in nsirdfcompositedatasource db, in nsrdfsortstate sortstateptr, in nsicontent root, in nsicontent trueparent, in nsicontent container, in nsicontent node, in boolean anotify ); parameters db sortstateptr root trueparent container node anotify sort() sort the contents of the widget containing anode using asortkey as the comparison key, and asorthints as how to sort.

database - Archive of obsolete content
« xul reference database type: nsirdfcompositedatasource returns the composite datasource created when all the datasources of an element are combined.

Introduction - Archive of obsolete content
when multiple datasources are used, an nsirdfcompositedatasource is created to hold all of the datasources.

Template Builder Interface - Archive of obsolete content
it implements the nsirdfcompositedatasource interface.

nsIDOMXULElement
database nsirdfcompositedatasource read only.

nsIXULTemplateBuilder
database nsirdfcompositedatasource readonly: the composite datasource that the template builder observes and uses to create content.