python-gitlab
diff --git a/‎gitlab/base.py‎
Lines changed: 14 additions & 1 deletion b/‎gitlab/base.py‎
Lines changed: 14 additions & 1 deletion
diff --git a/‎gitlab/mixins.py‎
Lines changed: 14 additions & 2 deletions b/‎gitlab/mixins.py‎
Lines changed: 14 additions & 2 deletions
diff --git a/‎gitlab/v4/objects/merge_trains.py‎
Lines changed: 1 addition & 0 deletions b/‎gitlab/v4/objects/merge_trains.py‎
Lines changed: 1 addition & 0 deletions
@@ -7,7 +7,7 @@
 import textwrap
 from collections.abc import Iterable
 from types import ModuleType
-from typing import Any, ClassVar, Generic, TYPE_CHECKING, TypeVar
+from typing import Any, cast, ClassVar, Generic, TYPE_CHECKING, TypeVar
 
 import gitlab
 from gitlab import types as g_types
@@ -351,6 +351,7 @@ class RESTManager(Generic[TObjCls]):
     _path: ClassVar[str]
     _obj_cls: type[TObjCls]
     _from_parent_attrs: dict[str, Any] = {}
+    _parent_ref_attr: ClassVar[str | None] = None
     _types: dict[str, type[g_types.GitlabAttribute]] = {}
 
     _computed_path: str
@@ -389,6 +390,18 @@ def _compute_path(self, path: str | None = None) -> str:
         self._parent_attrs = data
         return path.format(**data)
 
+    def _get_parent_ref_id(self) -> int | str | None:
+        if self._parent is None or not self._parent_ref_attr:
+            return None
+        if not hasattr(self._parent, self._parent_ref_attr):
+            return None
+        parent_ref = getattr(self._parent, self._parent_ref_attr)
+        if parent_ref is None:
+            return None
+        if hasattr(parent_ref, "iid"):
+            return cast(int, parent_ref.iid)
+        return None
+
     @property
     def path(self) -> str:
         return self._computed_path
@@ -72,11 +72,14 @@ class GetMixin(HeadMixin[base.TObjCls]):
     _optional_get_attrs: tuple[str, ...] = ()
 
     @exc.on_http_error(exc.GitlabGetError)
-    def get(self, id: str | int, lazy: bool = False, **kwargs: Any) -> base.TObjCls:
+    def get(
+        self, id: str | int | None = None, lazy: bool = False, **kwargs: Any
+    ) -> base.TObjCls:
         """Retrieve a single object.
 
         Args:
-            id: ID of the object to retrieve
+            id: ID of the object to retrieve. If not provided, falls back to
+                _parent_ref_id from the parent object (if available).
             lazy: If True, don't request the server, but create a
                          shallow object giving access to the managers. This is
                          useful if you want to avoid useless calls to the API.
@@ -89,6 +92,13 @@ def get(self, id: str | int, lazy: bool = False, **kwargs: Any) -> base.TObjCls:
             GitlabAuthenticationError: If authentication is not correct
             GitlabGetError: If the server cannot perform the request
         """
+        if id is None:
+            id = self._get_parent_ref_id()
+        if id is None:
+            raise ValueError(
+                "id is required. Either provide it explicitly or set "
+                "_parent_ref_attr on the manager to use the parent's reference."
+            )
         if isinstance(id, str):
             id = utils.EncodedId(id)
         path = f"{self.path}/{id}"
@@ -310,6 +320,8 @@ def update(
         """
         new_data = new_data or {}
 
+        if id is None:
+            id = self._get_parent_ref_id()
         if id is None:
             path = self.path
         else:
 
@@ -22,6 +22,7 @@ class ProjectMergeTrainMergeRequestManager(
     _path = "/projects/{project_id}/merge_trains/merge_requests"
     _obj_cls = ProjectMergeTrainMergeRequest
     _from_parent_attrs = {"project_id": "project_id"}
+    _parent_ref_attr = "merge_request"
     _update_method: UpdateMethod = UpdateMethod.POST
 
     _update_attrs = RequiredOptional(