Fix maps as well

serde_arrow/src/internal/deserialization/map_deserializer.rs

@@ -24,13 +24,19 @@ pub struct MapDeserializer<'a> {
 impl<'a> MapDeserializer<'a> {
     pub fn new(
         path: String,
-        key: ArrayDeserializer<'a>,
-        value: ArrayDeserializer<'a>,
+        mut key: ArrayDeserializer<'a>,
+        mut value: ArrayDeserializer<'a>,
         offsets: &'a [i32],
         validity: Option<BitsWithOffset<'a>>,
     ) -> Result<Self> {
         check_supported_list_layout(validity, offsets)?;
 
+        // skip any unused values up front
+        for _ in 0..usize::try_from(offsets[0])? {
+            key.deserialize_ignored_any(serde::de::IgnoredAny)?;
+            value.deserialize_ignored_any(serde::de::IgnoredAny)?;
+        }
+
         Ok(Self {
             path,
             key: Box::new(key),

serde_arrow/src/test_with_arrow/issue_248_sliced_lists.rs

@@ -1,4 +1,7 @@
+use std::collections::HashMap;
+
 use crate::_impl::arrow::datatypes::FieldRef;
+use crate::internal::testing::hash_map;
 use crate::{
     self as serde_arrow,
     schema::{SchemaLike, TracingOptions},
@@ -41,3 +44,24 @@ fn vec_of_strings() {
     let actual: Vec<Struct> = serde_arrow::from_record_batch(&batch).unwrap();
     assert_eq!(data[5..10], actual);
 }
+
+#[test]
+fn map_of_strings() {
+    #[derive(serde::Deserialize, serde::Serialize, Debug, PartialEq)]
+    struct Struct {
+        string: HashMap<String, String>,
+    }
+    let data = (1..=10)
+        .map(|x| Struct {
+            string: hash_map!(x.to_string() => x.to_string()),
+        })
+        .collect::<Vec<_>>();
+    let fields =
+        Vec::<FieldRef>::from_type::<Struct>(TracingOptions::default().map_as_struct(false))
+            .unwrap();
+    let batch = serde_arrow::to_record_batch(&fields, &data).unwrap();
+    let batch = batch.slice(5, 5);
+
+    let actual: Vec<Struct> = serde_arrow::from_record_batch(&batch).unwrap();
+    assert_eq!(data[5..10], actual);
+}