fix: make doc.save work and empty tables

frappe/model/document.py

@@ -1983,7 +1983,7 @@ class LazyChildTable:
 		self.fieldname = fieldname
 		self.doctype = doctype
 
-	def __get__(self, doc, objtype=None):
+	def __get__(self, doc: Document, objtype=None):
 		# TODO: review cached_property magic
 		children = frappe.db.sql(
 			"""SELECT * FROM {table_name}
@@ -1998,5 +1998,6 @@ class LazyChildTable:
 		)
 
 		# Update __dict__ and convert to Document objects
+		doc.__dict__[self.fieldname] = []
 		doc.extend(self.fieldname, children or [])
 		return doc.__dict__[self.fieldname]  # Note: avoid any high level access here

frappe/tests/test_document.py

@@ -541,6 +541,10 @@ class TestDocument(IntegrationTestCase):
 			self.assertEqual(guest_role.role, "Guest")
 			self.assertIsInstance(guest_role, type(eager_guest.roles[0]))
 
+		# Only one query for one table access
+		with self.assertQueryCount(1):
+			_ = guest.role_profiles
+
 		# No queries for repeat access, same object
 		with self.assertQueryCount(0):
 			guest_role_repeat_access = guest.roles[0]
@@ -552,6 +556,8 @@ class TestDocument(IntegrationTestCase):
 		# things accessing __dict__ by default should be updated too
 		self.assertTrue(frappe.get_lazy_doc("User", "Guest").get("roles"))
 
+		guest.save()
+
 
 class TestDocumentWebView(IntegrationTestCase):
 	def get(self, path, user="Guest"):