fix nested refs not being recognized as "populated" during serialization

document.go

@@ -84,6 +84,7 @@ type IDocument interface {
 	markPopulated(field string)
 	markDepopulated(field string)
 	newPopulationMap()
+	getPopulated() map[string]bool
 	getRaw() any
 	setRaw(raw any)
 }

document_internals.go

@@ -84,6 +84,14 @@ func serializeIDs(input interface{}, isJson bool, populated map[string]bool, par
 						break
 					}
 				}
+				if iidoc, ok := input.(IDocument); ok && !ip {
+					for k1, v1 := range iidoc.getPopulated() {
+						if k1 == descent || k1 == ft.Name {
+							ip = v1
+							break
+						}
+					}
+				}
 				if terr == nil {
 					ifc, ok := fv.Interface().(HasID)
 					if fv.Kind() == reflect.Slice {
@@ -316,6 +324,10 @@ func (d *Document) newPopulationMap() {
 	}
 }
 
+func (d *Document) getPopulated() map[string]bool {
+	return d.populatedFields
+}
+
 func (d *Document) setRaw(raw any) {
 	d.raw = raw
 }