[core] Render an id constant in an apply field as the object it names (#19694)

esphome/automation.py

@@ -239,9 +239,9 @@ class ApplyCall:
 
     Each arg is ``(conf_key, type_)`` or ``(conf_key, type_, const_fn)``. A ``conf_key`` may be a
     path into nested sections. A plain ``str`` ``type_`` is raw C++ type text and may use
-    ``{parent}``. ``const_fn(config, value)`` renders a constant's argument text; a lambda bypasses
-    it. The statement is skipped when none of its keys is set, always emitted when it has no
-    keys, and a partial set is a config error.
+    ``{parent}``. ``const_fn(config, value)`` renders a constant's argument text; a lambda or an
+    id bypasses it. The statement is skipped when none of its keys is set, always emitted when it
+    has no keys, and a partial set is a config error.
     """
 
     target: str
@@ -280,8 +280,8 @@ class ApplyField:
     Double a literal brace in a template. ``conf_key`` may be a path into nested sections.
     ``type_`` may be a C++ type string using ``{parent}`` when the type is only known per
     instance. ``const_fn(config, value)`` renders a constant's argument text when ``cg.safe_exp``
-    is not the right spelling (unit conversion belongs in the validator); a lambda bypasses it,
-    so the target must also take a plain ``type_``. An absent key emits nothing.
+    is not the right spelling (unit conversion belongs in the validator); a lambda or an id
+    bypasses it, so the target must also take a plain ``type_``. An absent key emits nothing.
     """
 
     conf_key: str | tuple[str, ...]
@@ -380,6 +380,9 @@ async def _render_values(
             expr = call_lambda(inner)
             bare = compare and isinstance(expr, cg.RawExpression)
             exprs.append(f"({expr})" if bare else str(expr))
+        elif isinstance(value, ID):
+            # Qualified like the parent, so a trigger arg named like the id cannot shadow it.
+            exprs.append(f"::{await cg.get_variable(value)}")
         elif const_fn is not None:
             exprs.append(const_fn(config, value))
         else:
@@ -402,6 +405,7 @@ def register_apply_action(
 
     Generates one stateless function for ``ApplyAction<Ts...>``: the parent (read from
     ``id_key``) and constants are baked in, lambdas are called inline with the trigger args.
+    A constant that is an id (``cv.use_id`` under ``cv.templatable``) is the object it names.
     With ``call`` every statement targets the call object ``auto apply_call = parent->call()``,
     and ``apply_call.perform()`` is appended.
     """

tests/unit_tests/test_automation.py

@@ -705,6 +705,32 @@ async def test_apply_constants(
     assert "set_ki" not in text
 
 
+@pytest.mark.asyncio
+async def test_apply_id_constant_is_the_named_object(
+    registries: tuple[Registry, Registry], mock_cg: MockCodegen
+) -> None:
+    """A templatable use_id given as a plain id renders the object it names."""
+    target = MockObj("speaker_b", "->")
+    mock_cg.get_variable.side_effect = [PARENT_OBJ, target]
+    fields = (ApplyField("target", "switch_to_output", cg.RawExpression("Speaker *")),)
+    await _run_apply_action(registries, fields, {"target": ID("speaker_b")})
+    mock_cg.get_variable.assert_any_await(ID("speaker_b"))
+    assert f"::{PARENT_OBJ}->switch_to_output(::speaker_b);" in _apply_lambda(mock_cg)
+
+
+@pytest.mark.asyncio
+async def test_apply_condition_id_constant_is_the_named_object(
+    registries: tuple[Registry, Registry], mock_cg: MockCodegen
+) -> None:
+    """The condition path resolves and qualifies an id constant the same way."""
+    target = MockObj("speaker_b", "->")
+    mock_cg.get_variable.side_effect = [PARENT_OBJ, target]
+    check = ApplyCall("is_output({})", (("target", cg.RawExpression("Speaker *")),))
+    await _run_apply_condition(registries, check, {"target": ID("speaker_b")})
+    mock_cg.get_variable.assert_any_await(ID("speaker_b"))
+    assert f"return ::{PARENT_OBJ}->is_output(::speaker_b);" in _apply_lambda(mock_cg)
+
+
 @pytest.mark.asyncio
 async def test_apply_lambdas(
     registries: tuple[Registry, Registry], mock_cg: MockCodegen