feat: StructAccessor.get returns Result<Option<Datum>> instead of Result<Datum> (#390)

This is so that the accessor's result can represent null field values.

Fixes: #379

Author: sdd

crates/iceberg/src/expr/accessor.rs

@@ -15,11 +15,12 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use crate::spec::{Datum, Literal, PrimitiveType, Struct};
-use crate::{Error, ErrorKind};
 use serde_derive::{Deserialize, Serialize};
 use std::sync::Arc;
 
+use crate::spec::{Datum, Literal, PrimitiveType, Struct};
+use crate::{Error, ErrorKind, Result};
+
 #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Eq)]
 pub struct StructAccessor {
     position: usize,
@@ -54,11 +55,13 @@ impl StructAccessor {
         &self.r#type
     }
 
-    pub(crate) fn get<'a>(&'a self, container: &'a Struct) -> crate::Result<Datum> {
+    pub(crate) fn get<'a>(&'a self, container: &'a Struct) -> Result<Option<Datum>> {
         match &self.inner {
             None => {
-                if let Literal::Primitive(literal) = &container[self.position] {
-                    Ok(Datum::new(self.r#type().clone(), literal.clone()))
+                if container.is_null_at_index(self.position) {
+                    Ok(None)
+                } else if let Literal::Primitive(literal) = &container[self.position] {
+                    Ok(Some(Datum::new(self.r#type().clone(), literal.clone())))
                 } else {
                     Err(Error::new(
                         ErrorKind::Unexpected,
@@ -95,7 +98,19 @@ mod tests {
         let test_struct =
             Struct::from_iter(vec![Some(Literal::bool(false)), Some(Literal::bool(true))]);
 
-        assert_eq!(accessor.get(&test_struct).unwrap(), Datum::bool(true));
+        assert_eq!(accessor.get(&test_struct).unwrap(), Some(Datum::bool(true)));
+    }
+
+    #[test]
+    fn test_single_level_accessor_null() {
+        let accessor = StructAccessor::new(1, PrimitiveType::Boolean);
+
+        assert_eq!(accessor.r#type(), &PrimitiveType::Boolean);
+        assert_eq!(accessor.position(), 1);
+
+        let test_struct = Struct::from_iter(vec![Some(Literal::bool(false)), None]);
+
+        assert_eq!(accessor.get(&test_struct).unwrap(), None);
     }
 
     #[test]
@@ -115,6 +130,25 @@ mod tests {
             Some(Literal::Struct(nested_test_struct)),
         ]);
 
-        assert_eq!(accessor.get(&test_struct).unwrap(), Datum::bool(true));
+        assert_eq!(accessor.get(&test_struct).unwrap(), Some(Datum::bool(true)));
+    }
+
+    #[test]
+    fn test_nested_accessor_null() {
+        let nested_accessor = StructAccessor::new(0, PrimitiveType::Boolean);
+        let accessor = StructAccessor::wrap(2, Box::new(nested_accessor));
+
+        assert_eq!(accessor.r#type(), &PrimitiveType::Boolean);
+        //assert_eq!(accessor.position(), 1);
+
+        let nested_test_struct = Struct::from_iter(vec![None, Some(Literal::bool(true))]);
+
+        let test_struct = Struct::from_iter(vec![
+            Some(Literal::bool(false)),
+            Some(Literal::bool(false)),
+            Some(Literal::Struct(nested_test_struct)),
+        ]);
+
+        assert_eq!(accessor.get(&test_struct).unwrap(), None);
     }
 }

crates/iceberg/src/spec/schema.rs

@@ -1683,39 +1683,39 @@ table {
                 .unwrap()
                 .get(&test_struct)
                 .unwrap(),
-            Datum::string("foo value")
+            Some(Datum::string("foo value"))
         );
         assert_eq!(
             schema
                 .accessor_by_field_id(2)
                 .unwrap()
                 .get(&test_struct)
                 .unwrap(),
-            Datum::int(1002)
+            Some(Datum::int(1002))
         );
         assert_eq!(
             schema
                 .accessor_by_field_id(3)
                 .unwrap()
                 .get(&test_struct)
                 .unwrap(),
-            Datum::bool(true)
+            Some(Datum::bool(true))
         );
         assert_eq!(
             schema
                 .accessor_by_field_id(16)
                 .unwrap()
                 .get(&test_struct)
                 .unwrap(),
-            Datum::string("Testy McTest")
+            Some(Datum::string("Testy McTest"))
         );
         assert_eq!(
             schema
                 .accessor_by_field_id(17)
                 .unwrap()
                 .get(&test_struct)
                 .unwrap(),
-            Datum::int(33)
+            Some(Datum::int(33))
         );
     }
 

crates/iceberg/src/spec/values.rs

@@ -1440,6 +1440,11 @@ impl Struct {
             },
         )
     }
+
+    /// returns true if the field at position `index` is null
+    pub fn is_null_at_index(&self, index: usize) -> bool {
+        self.null_bitmap[index]
+    }
 }
 
 impl Index<usize> for Struct {