feat(12.2-03): defer relation work on unsaved records and add child file routes

- record id 0 with X-Session-Key manages deferrable relations: create, link, unlink, delete and pivot edits are held in deferred_bindings
- the record's create save applies relation bindings with the file bindings; an ineligible link is a 422 on the relation-manager field
- child forms upload files through .../records/{child}/files/{field} keyed by X-Child-Session-Key; the child save commits them
- boot refuses a deferrable relation with create whose related model no plugin lists in Models()

modules/cabana/http.go

@@ -108,6 +108,13 @@ func Activate(app *backpack.App, plugins []party.Plugin) (*Routes, error) {
 	if err := checkFileLimits(reg, uploadBytes); err != nil {
 		return nil, err
 	}
+	var gdb *gorm.DB
+	if app != nil {
+		gdb, _ = app.Lookup[*gorm.DB]()
+	}
+	if err := checkDeferredModels(reg, plugins, gdb); err != nil {
+		return nil, err
+	}
 	if err := compileContributions(reg, plugins); err != nil {
 		return nil, err
 	}
@@ -291,6 +298,23 @@ func (s *service) mount(r pact.Router) {
 		constrainChild(g)
 		g.Put("/{vendor}/{plugin}/{controller}/{id}/relations/{name}/pivot/{child}", requireAjax(s.relationPivotUpdate))
 		constrainChild(g)
+		// File routes of a relation child form (D-17): {child} 0 is the
+		// child being created in the X-Child-Session-Key session.
+		const childFiles = "/{vendor}/{plugin}/{controller}/{id}/relations/{name}/records/{child}/files/{field}"
+		g.Get(childFiles, s.relationChildFileList)
+		constrainChildFile(g)
+		g.Post(childFiles, requireAjax(s.relationChildFileUpload))
+		constrainChildFile(g)
+		g.Post(childFiles+"/reorder", requireAjax(s.relationChildFileReorder))
+		constrainChildFile(g)
+		g.Put(childFiles+"/{file}", requireAjax(s.relationChildFileUpdate))
+		constrainChildFileID(g)
+		g.Delete(childFiles+"/{file}", requireAjax(s.relationChildFileRemove))
+		constrainChildFileID(g)
+		g.Get(childFiles+"/{file}/download", s.relationChildFileDownload)
+		constrainChildFileID(g)
+		g.Get(childFiles+"/{file}/thumb", s.relationChildFileThumb)
+		constrainChildFileID(g)
 		// File routes of `type: fileupload` fields (D-09). {id} 0 is the
 		// record being created in the X-Session-Key session.
 		g.Post("/{vendor}/{plugin}/{controller}/{id}/files/{field}", requireAjax(s.fileUpload))
@@ -335,6 +359,16 @@ func constrainChild(g pact.Router) {
 	g.Where("child", "[0-9]+")
 }
 
+func constrainChildFile(g pact.Router) {
+	constrainChild(g)
+	g.Where("field", "[A-Za-z_][A-Za-z0-9_]*")
+}
+
+func constrainChildFileID(g pact.Router) {
+	constrainChildFile(g)
+	g.Where("file", "[0-9]+")
+}
+
 func constrainFile(g pact.Router) {
 	constrainController(g)
 	g.Where("field", "[A-Za-z_][A-Za-z0-9_]*")
@@ -506,9 +540,8 @@ func (s *service) relationList(w http.ResponseWriter, r *http.Request, candidate
 			writeCRUDError(w, err)
 			return
 		}
-		svc, err := s.relations()
-		if err != nil {
-			WriteError(w, http.StatusInternalServerError, "error", msgServerError)
+		svc, ok := s.relationsFor(w, r)
+		if !ok {
 			return
 		}
 		var result *RelationResult
@@ -559,9 +592,8 @@ func (s *service) relationMutation(w http.ResponseWriter, r *http.Request, link
 			writeCRUDError(w, err)
 			return
 		}
-		svc, err := s.relations()
-		if err != nil {
-			WriteError(w, http.StatusInternalServerError, "error", msgServerError)
+		svc, ok := s.relationsFor(w, r)
+		if !ok {
 			return
 		}
 		var result RelationMutationResult
@@ -601,6 +633,25 @@ func (s *service) relations() (RelationService, error) {
 	return RelationService{DB: db, bucket: s.bucket(), tr: s.translator()}, nil
 }
 
+// relationsFor is the relation service of one request, carrying its
+// X-Session-Key: with it, record id 0 is the record being created in that
+// session. A malformed key is a 422; it writes the response and returns
+// false on failure.
+func (s *service) relationsFor(w http.ResponseWriter, r *http.Request) (RelationService, bool) {
+	key, _, err := sessionKeyFrom(r)
+	if err != nil {
+		writeCRUDError(w, err)
+		return RelationService{}, false
+	}
+	svc, err := s.relations()
+	if err != nil {
+		WriteError(w, http.StatusInternalServerError, "error", msgServerError)
+		return RelationService{}, false
+	}
+	svc.SessionKey = key
+	return svc, true
+}
+
 func (s *service) formSchema(w http.ResponseWriter, r *http.Request) {
 	s.protect(w, r, func(cc *CompiledController) {
 		if cc.Form == nil {