diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -204,7 +204,7 @@
     return hashlib.new("sha1", manifest).digest()
 
 
-class HashableObject(metaclass=ABCMeta):
+class BaseHashableModel(BaseModel, metaclass=ABCMeta):
     """Mixin to automatically compute object identifier hash when
     the associated model is instantiated."""
 
@@ -234,14 +234,14 @@
         return self.id
 
     def check(self) -> None:
-        super().check()  # type: ignore
+        super().check()
 
         if self.id != self.compute_hash():
             raise ValueError("'id' does not match recomputed hash.")
 
 
-class HashableObjectWithManifest(HashableObject):
-    """Derived class of HashableObject, for objects that may need to store
+class HashableObjectWithManifest(BaseHashableModel):
+    """Derived class of BaseHashableModel, for objects that may need to store
     verbatim git objects as ``raw_manifest`` to preserve original hashes."""
 
     __slots__ = ()
@@ -585,7 +585,7 @@
 
 
 @attr.s(frozen=True, slots=True)
-class Origin(HashableObject, BaseModel):
+class Origin(BaseHashableModel):
     """Represents a software source: a VCS and an URL."""
 
     object_type: Final = "origin"
@@ -728,7 +728,7 @@
 
 
 @attr.s(frozen=True, slots=True)
-class Snapshot(HashableObject, BaseModel):
+class Snapshot(BaseHashableModel):
     """Represents the full state of an origin at a given point in time."""
 
     object_type: Final = "snapshot"
@@ -1366,7 +1366,7 @@
 
 
 @attr.s(frozen=True, slots=True)
-class RawExtrinsicMetadata(HashableObject, BaseModel):
+class RawExtrinsicMetadata(BaseHashableModel):
     object_type: Final = "raw_extrinsic_metadata"
 
     # target object
@@ -1583,7 +1583,7 @@
 
 
 @attr.s(frozen=True, slots=True)
-class ExtID(HashableObject, BaseModel):
+class ExtID(BaseHashableModel):
     object_type: Final = "extid"
 
     extid_type = attr.ib(type=str, validator=type_validator())
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -1374,9 +1374,13 @@
 
 
 def test_object_type_is_final():
+    checked_classes = set()
     object_types = set()
 
     def check_final(cls):
+        if cls in checked_classes:
+            return
+        checked_classes.add(cls)
         if hasattr(cls, "object_type"):
             assert cls.object_type not in object_types
             object_types.add(cls.object_type)