Fix get_var_value silently returning wrong values for operation vars

news/6633.bugfix.md

@@ -0,0 +1 @@
+`State.get_var_value()` no longer silently returns a wrong value when passed a Var operation — an arithmetic/concatenation expression such as `State.a + State.b`, or an indexed/item access such as `State.items[0]`. Previously it resolved the state and field of the operation's *first* operand and returned that field's value instead of the operation's result. It now raises `UnretrievableVarValueError`, consistent with how it already handled vars not associated with any state. Plain field and computed-var references continue to resolve as before.

reflex/state.py

@@ -59,6 +59,7 @@
     ComputedVar,
     DynamicRouteVar,
     EvenMoreBasicBaseState,
+    ToOperation,
     Var,
     computed_var,
     dispatch,
@@ -1747,8 +1748,18 @@ async def get_var_value(self, var: Var[VAR_TYPE]) -> VAR_TYPE:
         ) is not unset and not isinstance(var_value, Var):
             return var_value  # pyright: ignore [reportReturnType]
 
-        var_data = var._get_all_var_data()
-        if var_data is None or not var_data.state:
+        # Unwrap any cast wrappers and resolve via the underlying var's *own*
+        # var data, not the recursive _get_all_var_data(). For an operation or
+        # derived var (e.g. State.a + State.b or State.items[0]), the recursive
+        # merge back-fills state/field_name from the first operand, which would
+        # make us silently return that operand's value instead of the operation's
+        # result. Only a plain field or computed var reference carries
+        # state + field_name on its own var data.
+        inner_var = var
+        while isinstance(inner_var, ToOperation):
+            inner_var = inner_var._original
+        var_data = inner_var._var_data
+        if var_data is None or not var_data.state or not var_data.field_name:
             msg = f"Unable to retrieve value for {var._js_expr}: not associated with any state."
             raise UnretrievableVarValueError(msg)
         # Fastish case: this var belongs to this state

tests/units/test_state.py

@@ -4575,6 +4575,21 @@ async def test_get_var_value(
         "b": [4, 5, 6],
     }
 
+    # Regression for https://github.com/reflex-dev/reflex/issues/6629: a Var
+    # operation / derived var (arithmetic, indexed or item access) must not
+    # silently return the value of its first constituent field. Such vars have
+    # no retrievable value, so raise instead of returning a plausible-but-wrong one.
+    with pytest.raises(UnretrievableVarValueError):
+        await state.get_var_value(TestState.num1 + TestState.num2)
+    with pytest.raises(UnretrievableVarValueError):
+        # array[0] is a Var operation at runtime, though statically typed as the element.
+        await state.get_var_value(TestState.array[0])  # pyright: ignore[reportArgumentType]
+    with pytest.raises(UnretrievableVarValueError):
+        await state.get_var_value(TestState.mapping["a"])
+
+    # Computed vars are derived but state-bound, so they remain resolvable.
+    assert await state.get_var_value(TestState.sum) == pytest.approx(42 + 3.15)
+
 
 @pytest.mark.asyncio
 async def test_get_var_value_async_computed_var(