fix: Support 'KEY' in fetch() for backward compatibility

In DataJoint 0.14, fetch('KEY') was a special syntax that returned
primary key columns as list of dicts. This was broken in 2.0.

Changes:
- fetch('KEY') returns list of primary key dicts
- fetch('KEY', 'col') returns (keys_list, col_array)
- fetch('KEY', as_dict=True) expands KEY to primary key columns

Fixes #1381

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/expression.py

@@ -639,12 +639,46 @@ def fetch(
 
         # Handle specific attributes requested
         if attrs:
+            # Check for special 'KEY' attribute
+            def is_key(attr):
+                return attr == "KEY"
+
+            has_key = any(is_key(a) for a in attrs)
+
+            # Handle fetch('KEY') alone - return list of primary key dicts
+            if has_key and len(attrs) == 1:
+                return list(self.keys(order_by=order_by, limit=limit, offset=offset))
+
             if as_dict is True:
                 # fetch('col1', 'col2', as_dict=True) -> list of dicts
-                return self.proj(*attrs).to_dicts(order_by=order_by, limit=limit, offset=offset, squeeze=squeeze)
+                # Replace KEY with primary key columns
+                proj_attrs = []
+                for attr in attrs:
+                    if is_key(attr):
+                        proj_attrs.extend(self.primary_key)
+                    else:
+                        proj_attrs.append(attr)
+                return self.proj(*proj_attrs).to_dicts(order_by=order_by, limit=limit, offset=offset, squeeze=squeeze)
             else:
                 # fetch('col1', 'col2') or fetch('col1', 'col2', as_dict=False) -> tuple of arrays
                 # This matches DJ 1.x behavior where fetch('col') returns array(['alpha', 'beta'])
+                if has_key:
+                    # Need to handle KEY specially - it returns list of dicts, not array
+                    proj_attrs = []
+                    for attr in attrs:
+                        if is_key(attr):
+                            proj_attrs.extend(self.primary_key)
+                        else:
+                            proj_attrs.append(attr)
+                    dicts = self.proj(*proj_attrs).to_dicts(order_by=order_by, limit=limit, offset=offset, squeeze=squeeze)
+                    # Build result, with KEY returning list of dicts
+                    results = []
+                    for attr in attrs:
+                        if is_key(attr):
+                            results.append([{k: d[k] for k in self.primary_key} for d in dicts])
+                        else:
+                            results.append(np.array([d[attr] for d in dicts]))
+                    return results[0] if len(attrs) == 1 else tuple(results)
                 return self.to_arrays(*attrs, order_by=order_by, limit=limit, offset=offset, squeeze=squeeze)
 
         # Handle as_dict=True -> to_dicts()

tests/integration/test_fetch.py

@@ -457,3 +457,52 @@ def test_to_arrays_inhomogeneous_shapes_second_axis(schema_any):
     assert data[0].shape == (100,)
     assert data[1].shape == (1, 100)
     assert data[2].shape == (2, 100)
+
+
+def test_fetch_KEY(lang, languages):
+    """Test fetch('KEY') returns list of primary key dicts.
+
+    Regression test for https://github.com/datajoint/datajoint-python/issues/1381
+    """
+    import warnings
+
+    # Suppress deprecation warning for fetch
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", DeprecationWarning)
+
+        # fetch('KEY') should return list of primary key dicts
+        keys = lang.fetch("KEY")
+        assert isinstance(keys, list)
+        assert len(keys) == len(languages)
+        assert all(isinstance(k, dict) for k in keys)
+        # Primary key is (name, language)
+        assert all(set(k.keys()) == {"name", "language"} for k in keys)
+
+
+def test_fetch1_KEY(lang):
+    """Test fetch1('KEY') returns primary key dict.
+
+    Regression test for https://github.com/datajoint/datajoint-python/issues/1381
+    """
+    key = {"name": "Edgar", "language": "Japanese"}
+    result = (lang & key).fetch1("KEY")
+    assert isinstance(result, dict)
+    assert result == key
+
+
+def test_fetch_KEY_with_other_attrs(lang):
+    """Test fetch('KEY', 'name') returns (keys_list, name_array).
+
+    Regression test for https://github.com/datajoint/datajoint-python/issues/1381
+    """
+    import warnings
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", DeprecationWarning)
+
+        # fetch('KEY', 'name') should return tuple of (list of dicts, array)
+        keys, names = lang.fetch("KEY", "name")
+        assert isinstance(keys, list)
+        assert all(isinstance(k, dict) for k in keys)
+        assert isinstance(names, np.ndarray)
+        assert len(keys) == len(names)