Make model subscription follow the same reasoning as node subscription

CHANGES.rst

@@ -7,6 +7,9 @@
 
 - Added documentation for ``stnode``. [#316]
 
+- Make datamodels follow the same subscription pattern as the ``stnode`` based
+  objects. [#322]
+
 0.19.0 (2024-02-09)
 ===================
 

src/roman_datamodels/datamodels/_core.py

@@ -266,10 +266,10 @@ def __getattr__(self, attr):
     def __setitem__(self, key, value):
         if key.startswith("_"):
             raise ValueError("May not specify attributes/keys that start with _")
-        if hasattr(self._instance, key):
-            setattr(self._instance, key, value)
-        else:
-            self._instance._data[key] = value
+        self._instance[key] = value
+
+    def __getitem__(self, key):
+        return self._instance[key]
 
     def __iter__(self):
         return iter(self._instance)

tests/test_models.py

@@ -515,7 +515,19 @@ def test_add_model_attribute(tmp_path):
         readnoise2.new_attribute = 88
         assert readnoise2.new_attribute == 88
         with pytest.raises(ValidationError):
-            readnoise["data"] = "bad_data_value"
+            readnoise.data = "bad_data_value"
+
+
+def test_model_subscribable(tmp_path):
+    """
+    Test that __getitem__ exists
+    """
+    file_path = tmp_path / "testreadnoise.asdf"
+
+    utils.mk_readnoise(shape=(8, 8), filepath=file_path)
+    with datamodels.open(file_path) as readnoise:
+        assert readnoise["data"].shape == (8, 8)
+        assert readnoise.data is readnoise["data"]
 
 
 # Saturation tests