diff --git a/tools/schemacode/bidsschematools/tests/test_expressions.py b/tools/schemacode/bidsschematools/tests/test_expressions.py
index 3c32cda023..e2e0704b37 100644
--- a/tools/schemacode/bidsschematools/tests/test_expressions.py
+++ b/tools/schemacode/bidsschematools/tests/test_expressions.py
@@ -1,3 +1,7 @@
+from collections.abc import Mapping
+from functools import singledispatch
+from typing import Union
+
 import pytest
 from pyparsing.exceptions import ParseException
 
@@ -11,6 +15,7 @@
     RightOp,
     expression,
 )
+from ..types import Namespace
 
 
 def test_schema_expressions(schema_obj):
@@ -88,65 +93,93 @@ def test_expected_failures(expr):
         expression.parse_string(expr)
 
 
+def walk_schema(schema_obj, predicate):
+    for key, value in schema_obj.items():
+        if predicate(key, value):
+            yield key, value
+        if isinstance(value, Mapping):
+            for subkey, value in walk_schema(value, predicate):
+                yield f"{key}.{subkey}", value
+
+
 def test_valid_sidecar_field(schema_obj):
     """Check sidecar fields actually exist in the metadata listed in the schema.
 
     Test failures are usually due to typos.
     """
-    for rules, level in ((schema_obj.rules.checks, 3),):
-        keys = (key for key in rules.keys(level=level) if key.endswith("selectors"))
-        check_fields(schema_obj, rules, keys)
-
-        keys = (key for key in rules.keys(level=level) if key.endswith("checks"))
-        check_fields(schema_obj, rules, keys)
-
-
-def check_fields(schema_obj, rules, keys):
-    for key in keys:
-        for rule in rules[key]:
-            ast = expression.parse_string(rule)[0]
-            if isinstance(ast, BinOp):
-                check_binop(schema_obj, ast)
-            elif isinstance(ast, Function):
-                check_function(schema_obj, ast)
-            elif isinstance(ast, Property):
-                check_property(schema_obj, ast)
-            elif isinstance(ast, RightOp):
-                check_half(schema_obj, ast.rh)
-
-
-def check_binop(schema_obj, binop):
-    for half in [binop.lh, binop.rh]:
-        check_half(schema_obj, half)
+    field_names = {field.name for key, field in schema_obj.objects.metadata.items()}
 
-
-def check_half(schema_obj, half):
-    if isinstance(half, BinOp):
-        check_binop(schema_obj, half)
-    elif isinstance(half, Function):
-        check_function(schema_obj, half)
-    elif isinstance(half, Property):
-        check_property(schema_obj, half)
-    elif isinstance(half, Element):
-        check_property(schema_obj, half.name)
-
-
-def check_function(schema_obj, function):
-    for x in function.args:
-        if isinstance(x, Property):
-            check_property(schema_obj, x)
-        elif isinstance(x, Function):
-            check_function(schema_obj, x)
-        elif isinstance(x, Array):
-            check_array(schema_obj, x)
-
-
-def check_array(schema_obj, array):
-    for element in array.elements:
-        if isinstance(element, Property):
-            check_property(schema_obj, element)
-
-
-def check_property(schema_obj, property):
-    if property.name == "sidecar":
-        assert property.field in schema_obj.objects.metadata
+    for key, rule in walk_schema(
+        schema_obj.rules, lambda k, v: isinstance(v, Mapping) and v.get("selectors")
+    ):
+        for selector in rule["selectors"]:
+            ast = expression.parse_string(selector)[0]
+            for name in find_names(ast):
+                if name.startswith(("json.", "sidecar.")):
+                    assert (
+                        name.split(".", 1)[1] in field_names
+                    ), f"Bad field in selector: {name} ({key})"
+        for selector in rule.get("checks", []):
+            ast = expression.parse_string(selector)[0]
+            for name in find_names(ast):
+                if name.startswith(("json.", "sidecar.")):
+                    assert (
+                        name.split(".", 1)[1] in field_names
+                    ), f"Bad field in selector: {name} ({key})"
+
+
+def test_test_valid_sidecar_field():
+    schema_obj = Namespace.build(
+        {
+            "objects": {
+                "metadata": {
+                    "a": {"name": "a"},
+                }
+            },
+            "rules": {"myruleA": {"selectors": ["sidecar.a"], "checks": ["json.a == sidecar.a"]}},
+        }
+    )
+    test_valid_sidecar_field(schema_obj)
+
+    schema_obj.objects.metadata.a["name"] = "b"
+    with pytest.raises(AssertionError):
+        test_valid_sidecar_field(schema_obj)
+
+
+@singledispatch
+def find_names(node: Union[ASTNode, str]):
+    # Walk AST nodes
+    if isinstance(node, BinOp):
+        yield from find_names(node.lh)
+        yield from find_names(node.rh)
+    elif isinstance(node, RightOp):
+        yield from find_names(node.rh)
+    elif isinstance(node, Array):
+        for element in node.elements:
+            yield from find_names(element)
+    elif isinstance(node, Element):
+        yield from find_names(node.name)
+        yield from find_names(node.index)
+    elif isinstance(node, (int, float)):
+        return
+    else:
+        raise TypeError(f"Unexpected node type: {node!r}")
+
+
+@find_names.register
+def find_function_names(node: Function):
+    yield node.name
+    for arg in node.args:
+        yield from find_names(arg)
+
+
+@find_names.register
+def find_property_name(node: Property):
+    # Properties are left-associative, so expand the left side
+    yield f"{next(find_names(node.name))}.{node.field}"
+
+
+@find_names.register
+def find_identifiers(node: str):
+    if not node.startswith(('"', "'")):
+        yield node