fix: Fix gather len in group-by state (#19586)

crates/polars-expr/src/expressions/gather.rs

@@ -84,7 +84,7 @@ impl PhysicalExpr for GatherExpr {
         };
 
         ac.with_series(taken.into_series(), true, Some(&self.expr))?;
-        ac.with_update_groups(UpdateGroups::WithGroupsLen);
+        ac.with_update_groups(UpdateGroups::WithSeriesLen);
         Ok(ac)
     }
 

py-polars/tests/unit/operations/test_gather.py

@@ -201,3 +201,15 @@ def test_gather_array_outer_validity_19482() -> None:
     expect = pl.Series([[1], None], dtype=pl.Array(pl.Int64, 1))
     assert_series_equal(s, expect)
     assert_series_equal(s.gather([0, 1]), expect)
+
+
+def test_gather_len_19561() -> None:
+    N = 4
+    df = pl.DataFrame({"foo": ["baz"] * N, "bar": range(N)})
+    idxs = pl.int_range(1, N).repeat_by(pl.int_range(1, N)).flatten()
+    gather = pl.col.bar.gather(idxs).alias("gather")
+
+    assert df.group_by("foo").agg(gather.len()).to_dict(as_series=False) == {
+        "foo": ["baz"],
+        "gather": [6],
+    }