fix: Force flattened record according to provided flattened schema

singer_sdk/helpers/_flattening.py

@@ -415,7 +415,10 @@ def _flatten_record(
     items: list[tuple[str, t.Any]] = []
     for k, v in record_node.items():
         new_key = flatten_key(k, parent_key, separator)
-        if isinstance(v, collections.abc.MutableMapping) and level < max_level:
+        if isinstance(v, collections.abc.MutableMapping) and (
+            (new_key not in flattened_schema.get("properties", {}))
+            and (level < max_level)
+        ):
             items.extend(
                 _flatten_record(
                     v,

tests/core/test_flattening.py

@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+import pytest
+
+from singer_sdk.helpers._flattening import flatten_record
+
+
+@pytest.mark.parametrize(
+    "flattened_schema, max_level, expected",
+    [
+        pytest.param(
+            {
+                "properties": {
+                    "key_1": {"type": ["null", "integer"]},
+                    "key_2__key_3": {"type": ["null", "string"]},
+                    "key_2__key_4": {"type": ["null", "object"]},
+                }
+            },
+            99,
+            {
+                "key_1": 1,
+                "key_2__key_3": "value",
+                "key_2__key_4": '{"key_5": 1, "key_6": ["a", "b"]}',
+            },
+            id="flattened schema limiting the max level",
+        ),
+        pytest.param(
+            {
+                "properties": {
+                    "key_1": {"type": ["null", "integer"]},
+                    "key_2__key_3": {"type": ["null", "string"]},
+                    "key_2__key_4__key_5": {"type": ["null", "integer"]},
+                    "key_2__key_4__key_6": {"type": ["null", "array"]},
+                }
+            },
+            99,
+            {
+                "key_1": 1,
+                "key_2__key_3": "value",
+                "key_2__key_4__key_5": 1,
+                "key_2__key_4__key_6": '["a", "b"]',
+            },
+            id="flattened schema not limiting the max level",
+        ),
+        pytest.param(
+            {
+                "properties": {
+                    "key_1": {"type": ["null", "integer"]},
+                    "key_2__key_3": {"type": ["null", "string"]},
+                    "key_2__key_4__key_5": {"type": ["null", "integer"]},
+                    "key_2__key_4__key_6": {"type": ["null", "array"]},
+                }
+            },
+            1,
+            {
+                "key_1": 1,
+                "key_2__key_3": "value",
+                "key_2__key_4": '{"key_5": 1, "key_6": ["a", "b"]}',
+            },
+            id="max level limiting flattened schema",
+        ),
+    ],
+)
+def test_flatten_record(flattened_schema, max_level, expected):
+    """Test flatten_record to obey the max_level and flattened_schema parameters."""
+    record = {
+        "key_1": 1,
+        "key_2": {"key_3": "value", "key_4": {"key_5": 1, "key_6": ["a", "b"]}},
+    }
+
+    result = flatten_record(
+        record, max_level=max_level, flattened_schema=flattened_schema
+    )
+    assert expected == result