fix: fetch('column') returns array instead of list of dicts

In DJ 1.x, fetch('column') returns an array like ['alpha', 'beta'].
The previous implementation incorrectly returned list of dicts when
as_dict was not specified.

Fix: Change condition from `if as_dict or as_dict is None` to
`if as_dict is True` so unspecified as_dict defaults to array output.

Before (broken):
  names = Table().fetch("name")  # [{'name': 'alpha'}, {'name': 'beta'}]
  "alpha" in names  # False

After (matches DJ 1.x):
  names = Table().fetch("name")  # array(['alpha', 'beta'])
  "alpha" in names  # True

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

Author: dimitri-yatsenko

src/datajoint/expression.py

@@ -639,11 +639,12 @@ def fetch(
 
         # Handle specific attributes requested
         if attrs:
-            if as_dict or as_dict is None:
-                # fetch('col1', 'col2', as_dict=True) or fetch('col1', 'col2')
+            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)
             else:
-                # fetch('col1', 'col2', as_dict=False) -> tuple of arrays
+                # 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'])
                 return self.to_arrays(*attrs, order_by=order_by, limit=limit, offset=offset, squeeze=squeeze)
 
         # Handle as_dict=True -> to_dicts()

tests/unit/test_fetch_compat.py

@@ -53,12 +53,23 @@ def test_fetch_as_dict_true(self, mock_expression):
 
         mock_expression.to_dicts.assert_called_once_with(order_by=None, limit=None, offset=None, squeeze=False)
 
-    def test_fetch_with_attrs_returns_dicts(self, mock_expression):
-        """fetch('col1', 'col2') should call proj().to_dicts()."""
+    def test_fetch_with_attrs_returns_arrays(self, mock_expression):
+        """fetch('col1', 'col2') should call to_arrays() - matches DJ 1.x behavior."""
         with warnings.catch_warnings():
             warnings.simplefilter("ignore", DeprecationWarning)
             mock_expression.fetch("col1", "col2")
 
+        # DJ 1.x: fetch('col') returns array(['alpha', 'beta']), not list of dicts
+        mock_expression.to_arrays.assert_called_once_with(
+            "col1", "col2", order_by=None, limit=None, offset=None, squeeze=False
+        )
+
+    def test_fetch_with_attrs_as_dict_true(self, mock_expression):
+        """fetch('col1', 'col2', as_dict=True) should call proj().to_dicts()."""
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
+            mock_expression.fetch("col1", "col2", as_dict=True)
+
         mock_expression.proj.assert_called_once_with("col1", "col2")
         mock_expression.to_dicts.assert_called_once()