@classmethod
     def by_id(cls, db_conn: DatabaseConnection, id_: int | None,
               create: bool = False) -> Process:
-        """Collect all Processes and their connected VersionedAttributes."""
+        """Collect Process, its VersionedAttributes, and its child IDs."""
         process = None
         for row in db_conn.exec('SELECT * FROM processes '
                                 'WHERE id = ?', (id_,)):
                 process.child_ids += [row[1]]
         return process
 
-    def children(self, db_conn: DatabaseConnection) -> list[Process]:
-        """Return child Processes as determined by self.child_ids."""
-        return [self.__class__.by_id(db_conn, id_) for id_ in self.child_ids]
+    def get_descendants(self, db_conn: DatabaseConnection) ->\
+            dict[int, dict[str, object]]:
+        """Return tree of descendant Processes"""
+        descendants = {}
+        for id_ in self.child_ids:
+            child = self.__class__.by_id(db_conn, id_)
+            descendants[id_] = {'process': child,
+                                'children': child.get_descendants(db_conn)}
+        return descendants
 
     def save(self, db_conn: DatabaseConnection) -> None:
-        """Add (or re-write) self and connected VersionedAttributes to DB."""
+        """Add (or re-write) self and connected VersionedAttributes to DB.
+
+        Also is the point at which descendancy recursion is checked.
+        """
+        def walk_descendants(node_id: int) -> None:
+            if node_id == self.id_:
+                raise BadFormatException('bad child selection: recursion')
+            descendant = self.by_id(db_conn, node_id)
+            for descendant_id in descendant.child_ids:
+                walk_descendants(descendant_id)
         cursor = db_conn.exec('REPLACE INTO processes VALUES (?)', (self.id_,))
         self.id_ = cursor.lastrowid
         self.title.save(db_conn)
         db_conn.exec('DELETE FROM process_children WHERE parent_id = ?',
                      (self.id_,))
         for child_id in self.child_ids:
+            walk_descendants(child_id)
             db_conn.exec('INSERT INTO process_children VALUES (?, ?)',
                          (self.id_, child_id))
 
 
         p_saved.save(self.db_conn)
         p_loaded = Process.by_id(self.db_conn, p_saved.id_)
         self.assertEqual(p_saved.title.history, p_loaded.title.history)
+        p_9 = Process(9)
+        p_9.child_ids = [4]
+        with self.assertRaises(NotFoundException):
+            p_9.save(self.db_conn)
+        p_9.child_ids = [5]
+        p_9.save(self.db_conn)
+        p_5 = Process.by_id(self.db_conn, 5)
+        p_5.child_ids = [1]
+        p_5.save(self.db_conn)
+        p_1 = Process.by_id(self.db_conn, 1)
+        p_1.child_ids = [9]
+        with self.assertRaises(BadFormatException):
+            p_1.save(self.db_conn)
 
     def test_Process_by_id(self) -> None:
         """Test Process.by_id()."""
 
     def test_do_POST_process(self) -> None:
         """Test POST /process and its effect on the database."""
-        form_data = {'title': 'foo', 'description': 'foo',
-                     'effort': 1.1, 'children': [1, 2]}
+        form_data = {'title': 'foo', 'description': 'foo', 'effort': 1.1}
+        self.check_post(form_data, '/process?id=', 302, '/')
         self.check_post(form_data, '/process?id=FOO', 400)
         form_data['effort'] = 'foo'
         self.check_post(form_data, '/process?id=', 400)
-        form_data['effort'] = 1.1
-        form_data['children'] = 1.1
-        self.check_post(form_data, '/process?id=', 400)
-        form_data['children'] = 'a'
-        self.check_post(form_data, '/process?id=', 400)
-        form_data['children'] = [1, 1.2]
-        self.check_post(form_data, '/process?id=', 400)
-        form_data['children'] = [1, 'b']
-        self.check_post(form_data, '/process?id=', 400)
         self.check_post({}, '/process?id=', 400)
         form_data = {'title': '', 'description': ''}
         self.check_post(form_data, '/process?id=', 400)
         form_data = {'description': '', 'effort': 1.0}
         self.check_post(form_data, '/process?id=', 400)
         form_data = {'title': '', 'description': '',
-                     'effort': 1.1, 'children': [1, 2]}
+                     'effort': 1.1, 'children': [1]}
         self.check_post(form_data, '/process?id=', 302, '/')
+        form_data['children'] = 1.1
+        self.check_post(form_data, '/process?id=', 400)
+        form_data['children'] = 'a'
+        self.check_post(form_data, '/process?id=', 400)
+        form_data['children'] = [1.2]
+        self.check_post(form_data, '/process?id=', 400)
+        form_data['children'] = ['b']
+        self.check_post(form_data, '/process?id=', 400)
+        form_data['children'] = [2]
+        self.check_post(form_data, '/process?id=1', 400)
         retrieved_1 = Process.by_id(self.db_conn, 1)
-        self.assertEqual(retrieved_1.title.newest, '')
-        self.assertEqual(retrieved_1.child_ids, [1, 2])
-        form_data['children'] = []
-        self.check_post(form_data, '/process?id=', 302, '/')
+        self.assertEqual(retrieved_1.title.newest, 'foo')
+        self.assertEqual(retrieved_1.child_ids, [])
         retrieved_2 = Process.by_id(self.db_conn, 2)
-        self.assertEqual(retrieved_2.child_ids, [])
-        del form_data['children']
-        self.check_post(form_data, '/process?id=', 302, '/')
-        retrieved_3 = Process.by_id(self.db_conn, 3)
-        self.assertEqual(retrieved_2.child_ids, [])
+        self.assertEqual(retrieved_2.child_ids, [1])
+        form_data = {'title': 'bar', 'description': 'bar', 'effort': 1.1}
         self.check_post(form_data, '/process?id=1', 302, '/')
-        self.assertEqual([p.id_ for p in Process.all(self.db_conn)],
-                         [retrieved_1.id_, retrieved_2.id_, retrieved_3.id_])
         retrieved_1 = Process.by_id(self.db_conn, 1)
-        self.assertEqual(retrieved_1.child_ids, [])
-        self.check_post(form_data, '/process', 302, '/')
+        self.assertEqual(retrieved_1.title.newest, 'bar')
+        self.assertEqual([p.id_ for p in Process.all(self.db_conn)],
+                         [retrieved_1.id_, retrieved_2.id_])
 
     def test_do_GET(self) -> None:
         """Test /process and /processes response codes."""