diff --git a/swh/model/from_disk.py b/swh/model/from_disk.py
--- a/swh/model/from_disk.py
+++ b/swh/model/from_disk.py
@@ -121,7 +121,7 @@
     """
 
     __slots__ = []  # type: List[str]
-    type = "content"
+    object_type: Final = "content"
 
     @classmethod
     def from_bytes(cls, *, mode, data):
@@ -300,7 +300,7 @@
     """
 
     __slots__ = ["__entries"]
-    type = "directory"
+    object_type: Final = "directory"
 
     @classmethod
     def from_disk(
@@ -352,14 +352,14 @@
 
     @staticmethod
     def child_to_directory_entry(name, child):
-        if isinstance(child, Directory):
+        if child.object_type == "directory":
             return {
                 "type": "dir",
                 "perms": DentryPerms.directory,
                 "target": child.hash,
                 "name": name,
             }
-        elif isinstance(child, Content):
+        elif child.object_type == "content":
             return {
                 "type": "file",
                 "perms": child.data["perms"],
@@ -367,7 +367,7 @@
                 "name": name,
             }
         else:
-            raise ValueError("unknown child")
+            raise ValueError(f"unknown child {child}")
 
     def get_data(self, **kwargs):
         return {
diff --git a/swh/model/merkle.py b/swh/model/merkle.py
--- a/swh/model/merkle.py
+++ b/swh/model/merkle.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2017 The Software Heritage developers
+# Copyright (C) 2017-2020 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -8,7 +8,7 @@
 import abc
 import collections
 
-from typing import Iterator, List, Optional, Set
+from typing import Iterator, List, Set
 
 
 def deep_update(left, right):
@@ -111,7 +111,6 @@
 
     __slots__ = ["parents", "data", "__hash", "collected"]
 
-    type = None  # type: Optional[str]  # TODO: make this an enum
     """Type of the current node (used as a classifier for :func:`collect`)"""
 
     def __init__(self, data=None):
@@ -234,7 +233,7 @@
         """
         if not self.collected:
             self.collected = True
-            return {self.type: {self.hash: self.get_data(**kwargs)}}
+            return {self.object_type: {self.hash: self.get_data(**kwargs)}}
         else:
             return {}
 
diff --git a/swh/model/tests/test_merkle.py b/swh/model/tests/test_merkle.py
--- a/swh/model/tests/test_merkle.py
+++ b/swh/model/tests/test_merkle.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2017 The Software Heritage developers
+# Copyright (C) 2017-2020 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -9,7 +9,7 @@
 
 
 class MerkleTestNode(merkle.MerkleNode):
-    type = "tested_merkle_node_type"
+    object_type = "tested_merkle_node_type"
 
     def __init__(self, data):
         super().__init__(data)
@@ -23,7 +23,7 @@
 
 
 class MerkleTestLeaf(merkle.MerkleLeaf):
-    type = "tested_merkle_leaf_type"
+    object_type = "tested_merkle_leaf_type"
 
     def __init__(self, data):
         super().__init__(data)
@@ -62,7 +62,11 @@
         collected = self.instance.collect()
         self.assertEqual(
             collected,
-            {self.instance.type: {self.instance.hash: self.instance.get_data(),},},
+            {
+                self.instance.object_type: {
+                    self.instance.hash: self.instance.get_data(),
+                },
+            },
         )
         collected2 = self.instance.collect()
         self.assertEqual(collected2, {})
@@ -162,7 +166,7 @@
 
     def test_collect(self):
         collected = self.root.collect()
-        self.assertEqual(len(collected[self.root.type]), len(self.nodes))
+        self.assertEqual(len(collected[self.root.object_type]), len(self.nodes))
         for node in self.nodes.values():
             self.assertTrue(node.collected)
         collected2 = self.root.collect()
@@ -229,7 +233,7 @@
         # Ensure we collected root, root/b, and both new children
         collected_after_update = self.root.collect()
         self.assertCountEqual(
-            collected_after_update[MerkleTestNode.type],
+            collected_after_update[MerkleTestNode.object_type],
             [
                 self.nodes[b"root"].hash,
                 self.nodes[b"root/b"].hash,