chore: Rename Doctype Test and more explicit comment

- Better decription of why the fix is done, what case it handles
- Test for Renaming Doctype and Record having same name as DocType

frappe/model/rename_doc.py

@@ -251,8 +251,14 @@ def update_link_field_values(link_fields, old, new, doctype):
 		else:
 			parent = field['parent']
 
-			# because the table hasn't been renamed yet!
-			if field['parent'] == new and doctype == "DocType":
+			# Handles the case where one of the link fields belongs to
+			# the DocType being renamed.
+			# Here this field could have the current DocType as its value too.
+
+			# In this case while updating link field value, the field's parent
+			# or the current DocType table name hasn't been renamed yet,
+			# so consider it's old name.
+			if parent == new and doctype == "DocType":
 				parent = old
 
 			frappe.db.sql("""

frappe/tests/test_document.py

@@ -288,4 +288,39 @@ class TestDocument(unittest.TestCase):
 		self.assertEqual(merged_todo_doc.priority, second_todo_doc.priority)
 
 		for docname in available_documents:
-			frappe.delete_doc(doctype, docname)
+			frappe.delete_doc(doctype, docname)
+
+	def test_rename_doctype(self):
+		from frappe.core.doctype.doctype.test_doctype import new_doctype
+
+		fields =[{
+			"label": "Linked To",
+			"fieldname": "linked_to_doctype",
+			"fieldtype": "Link",
+			"options": "DocType",
+			"unique": 0
+		}]
+		if not frappe.db.exists("DocType", "Rename This"):
+			new_doctype("Rename This", unique=0, fields=fields).insert()
+
+		to_rename_record = frappe.get_doc({
+			"doctype": "Rename This",
+			"linked_to_doctype": "Rename This"
+		})
+		to_rename_record.insert()
+
+		# Rename doctype
+		self.assertEqual("Renamed Doc", frappe.rename_doc("DocType", "Rename This", "Renamed Doc", force=True))
+
+		# Test if Doctype value has changed in Link field
+		renamed_doctype_record = frappe.get_doc("Renamed Doc", to_rename_record.name)
+		self.assertEqual(renamed_doctype_record.linked_to_doctype, "Renamed Doc")
+
+		# Test if there are conflicts between a record and a DocType
+		# having the same name
+		old_name = to_rename_record.name
+		new_name = "ToDo"
+		self.assertEqual(new_name, frappe.rename_doc("Renamed Doc", old_name, new_name, force=True))
+
+		frappe.delete_doc_if_exists("Renamed Doc", "ToDo")
+		frappe.delete_doc_if_exists("DocType", "Renamed Doc")