have Scalar.__getitem__ raise and instruct to use to_delayed

src/dask_awkward/lib/core.py

@@ -193,15 +193,11 @@ def __str__(self) -> str:
         return f"dask.awkward<{key_split(self.name)}, type=Scalar, dtype={dt}>"
 
     def __getitem__(self, where: Any) -> Any:
-        token = tokenize(self, operator.getitem, where)
-        label = "getitem"
-        name = f"{label}-{token}"
-        task = AwkwardMaterializedLayer(
-            {(name, 0): (operator.getitem, self.key, where)},
-            previous_layer_names=[self.name],
+        msg = (
+            "__getitem__ access on Scalars should be done after converting "
+            "the Scalar collection to delayed with the to_delayed method."
         )
-        hlg = HighLevelGraph.from_collections(name, task, dependencies=[self])
-        return new_scalar_object(hlg, name, meta=None)
+        raise RuntimeError(msg)
 
     @property
     def known_value(self) -> Any | None:

tests/test_core.py

@@ -206,7 +206,9 @@ def test_scalar_collection(daa: Array) -> None:
 def test_scalar_getitem_getattr() -> None:
     d = {"a": 5}
     s = new_known_scalar(d)
-    assert s["a"].compute() == d["a"]
+    with pytest.raises(RuntimeError, match="should be done after converting"):
+        s["a"].compute() == d["a"]
+    s.to_delayed()["a"].compute() == d["a"]
     Thing = namedtuple("Thing", "a b c")
     t = Thing(c=3, b=2, a=1)
     s = new_known_scalar(t)