fix struct list concat (#3435)

polars/polars-core/src/chunked_array/logical/struct_/mod.rs

@@ -42,6 +42,12 @@ impl StructChunked {
         &self.arrow_array
     }
 
+    pub fn rechunk(&mut self) {
+        let (arrow_array, fields) = fields_to_struct_array(&self.fields);
+        self.arrow_array = arrow_array;
+        self.fields = fields;
+    }
+
     /// Does not check the lengths of the fields
     pub(crate) fn new_unchecked(name: &str, fields: &[Series]) -> Self {
         let dtype = DataType::Struct(

polars/polars-core/src/series/implementations/struct_.rs

@@ -207,7 +207,9 @@ impl SeriesTrait for SeriesWrap<StructChunked> {
 
     /// Aggregate all chunks to a contiguous array of memory.
     fn rechunk(&self) -> Series {
-        self.0.apply_fields(|s| s.rechunk()).into_series()
+        let mut out = self.0.clone();
+        out.rechunk();
+        out.into_series()
     }
 
     fn expand_at_index(&self, index: usize, length: usize) -> Series {

polars/polars-ops/src/chunked_array/list/namespace.rs

@@ -217,7 +217,6 @@ pub trait ListNameSpaceImpl: AsList {
         let length = ca.len();
         let mut other = other.to_vec();
         let dtype = ca.dtype();
-        dbg!(&ca, ca.dtype());
         let inner_type = ca.inner_dtype();
 
         // broadcasting path in case all unit length
@@ -256,6 +255,13 @@ pub trait ListNameSpaceImpl: AsList {
                     for append in &to_append {
                         s.append(append).unwrap();
                     }
+                    match inner_type {
+                        // structs don't have chunks, so we must first rechunk the underlying series
+                        #[cfg(feature = "dtype-struct")]
+                        DataType::Struct(_) => s = s.rechunk(),
+                        // nothing
+                        _ => {}
+                    }
                     s
                 });
                 builder.append_opt_series(opt_s.as_ref())
@@ -310,6 +316,13 @@ pub trait ListNameSpaceImpl: AsList {
                         }
                     }
                 }
+                match inner_type {
+                    // structs don't have chunks, so we must first rechunk the underlying series
+                    #[cfg(feature = "dtype-struct")]
+                    DataType::Struct(_) => acc = acc.rechunk(),
+                    // nothing
+                    _ => {}
+                }
                 builder.append_series(&acc);
             }
             Ok(builder.finish())

py-polars/tests/test_struct.py

@@ -361,3 +361,23 @@ def test_struct_arr_methods() -> None:
     assert df.select([pl.col("list_struct").arr.get(0)]).to_dict(False) == {
         "list_struct": [{"a": 1, "b": 2}, {"a": 1, "b": 2}, {"a": 1, "b": 2}]
     }
+
+
+def test_struct_concat_list() -> None:
+    assert pl.DataFrame(
+        {
+            "list_struct1": [
+                [{"a": 1, "b": 2}, {"a": 3, "b": 4}],
+                [{"a": 1, "b": 2}],
+            ],
+            "list_struct2": [
+                [{"a": 6, "b": 7}, {"a": 8, "b": 9}],
+                [{"a": 6, "b": 7}],
+            ],
+        }
+    ).with_columns([pl.col("list_struct1").arr.concat("list_struct2").alias("result")])[
+        "result"
+    ].to_list() == [
+        [{"a": 1, "b": 2}, {"a": 3, "b": 4}, {"a": 6, "b": 7}, {"a": 8, "b": 9}],
+        [{"a": 1, "b": 2}, {"a": 6, "b": 7}],
+    ]