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

[dimitri-yatsenko]

Summary

Fix fetch('column') to return an array like DJ 1.x, not a list of dicts.

Problem

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

# DJ 2.0 broken behavior:
names = Table().fetch("name")  # [{'name': 'alpha'}, {'name': 'beta'}]
"alpha" in names  # False  ← breaks existing code!

Root Cause

In expression.py:642, the condition:

if as_dict or as_dict is None:

Treats unspecified as_dict the same as True, returning dicts instead of arrays.

Fix

Change to:

if as_dict is True:

So that unspecified as_dict defaults to array output, matching DJ 1.x.

Behavior After Fix

Pattern	Returns
fetch('col')	array (DJ 1.x compatible)
fetch('col', as_dict=False)	array
fetch('col', as_dict=True)	list of dicts

Testing

• Updated test_fetch_with_attrs_returns_arrays to verify correct behavior
• Added test_fetch_with_attrs_as_dict_true for explicit dict case
• All 10 fetch compat tests pass

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