fix: reject transform renames that collide column names

sqlite_utils/db.py

@@ -2998,6 +2998,26 @@ def fk_with_renamed_columns(fk: ForeignKey) -> ForeignKey:
             new_column_pairs.append((new_name, type_))
             copy_from_to[name] = new_name
 
+        duplicate_columns = sorted(
+            {
+                column_name
+                for column_name, _ in new_column_pairs
+                if sum(
+                    1 for other_name, _ in new_column_pairs if other_name == column_name
+                )
+                > 1
+            }
+        )
+        if duplicate_columns:
+            raise TransformError(
+                "Cannot transform table '{}': renaming or retyping columns would result "
+                "in duplicate column name{} '{}'. No changes have been applied to this table.".format(
+                    self.name,
+                    "s" if len(duplicate_columns) > 1 else "",
+                    "', '".join(duplicate_columns),
+                )
+            )
+
         if existing_autoincrement:
             existing_autoincrement = resolve_casing(
                 existing_autoincrement, existing_columns

tests/test_transform.py

@@ -254,6 +254,34 @@ def test_transform_rename_pk(fresh_db):
     )
 
 
+def test_transform_rename_cannot_lose_data_by_colliding_columns(fresh_db):
+    # Renaming one column onto the name of another used to silently collapse
+    # the two columns into one, discarding the source column's data entirely.
+    table = fresh_db.table("items")
+    table.insert({"a": 1, "b": 2})
+    with pytest.raises(TransformError, match="duplicate column name"):
+        table.transform(rename={"b": "a"})
+    # The failed transform must leave the table and its data untouched.
+    assert table.count == 1
+    assert [(r["a"], r["b"]) for r in table.rows_where(order_by="rowid")] == [(1, 2)]
+    assert '"b"' in table.schema
+
+
+def test_transform_rename_two_columns_to_same_name_errors(fresh_db):
+    table = fresh_db.table("items")
+    table.insert({"a": 1, "b": 2, "c": 3})
+    with pytest.raises(TransformError, match="duplicate column name 'x'"):
+        table.transform(rename={"b": "x", "c": "x"})
+
+
+def test_transform_can_swap_two_column_names(fresh_db):
+    # A straight swap a<->b is legal and must not be blocked.
+    table = fresh_db.table("items")
+    table.insert({"a": 1, "b": 2})
+    table.transform(rename={"a": "b", "b": "a"})
+    assert [(r["a"], r["b"]) for r in table.rows_where(order_by="rowid")] == [(2, 1)]
+
+
 def test_transform_preserves_keyword_literal_defaults(fresh_db):
     # transform() used to requote keyword-literal defaults (DEFAULT TRUE became
     # DEFAULT 'TRUE'), so a default insert stored the text 'TRUE' instead of the