apache/seatunnel · error · java.lang.UnsupportedOperationException

Map in list writing not yet implemented

Error message

Map in list writing not yet implemented

What it means

MapTypeWriter.writeToListWriter rejects MAP elements nested inside a LIST column. Writing map-typed elements of an Arrow list vector is not implemented in the Lance connector; the generic list writer dispatch reaches this method and throws UnsupportedOperationException.

Source

Thrown at seatunnel-connectors-v2/connector-lance/src/main/java/org/apache/seatunnel/connectors/seatunnel/lance/sink/writers/MapTypeWriter.java:41

import org.apache.arrow.vector.complex.impl.UnionMapWriter;
import org.apache.arrow.vector.types.pojo.ArrowType;

public class MapTypeWriter extends BaseTypeWriter {
    @Override
    public void writeToVector(
            FieldVector vector,
            ArrowType arrowType,
            Object value,
            int rowIndex,
            BufferAllocator allocator) {
        throw new UnsupportedOperationException(
                "Map type should be handled via FragmentConverter.writeMapToVector");
    }

    @Override
    public void writeToListWriter(
            UnionListWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("Map in list writing not yet implemented");
    }

    @Override
    public void writeToMapKey(
            UnionMapWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("Map as map key is not supported");
    }

    @Override
    public void writeToMapValue(
            UnionMapWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("Map as map value not yet implemented");
    }
}

View on GitHub (pinned to cf67b549a7)

Solutions

1. Flatten to a list of structs instead: array<struct<...>> which Lance/Arrow supports via the struct element writer.
2. Serialize each map element to a JSON string so the list element type is a scalar string.
3. Reshape data so maps live at the top level (handled by FragmentConverter.writeMapToVector) rather than inside lists.
4. Implement writeToListWriter for maps in a fork if nested support is required.

Example fix

// before
columns = "items array<map<string, string>>"

// after: list of structs
columns = "items array<struct<k string, v string>>"

// Java: reject list-of-map columns before writing
Field elementField = listField.getChildren().get(0);
if (elementField.getType() instanceof ArrowType.Map) {
    throw new IllegalArgumentException("array<map> unsupported; use array<struct> or JSON strings");
}

boolean isSupportedListElement(ArrowType t) {
    return !(t instanceof ArrowType.Map);
}

When it happens

Trigger: A SeaTunnel LIST column whose element type is MAP (e.g. array<map<string,string>>); when FragmentConverter.writeListElement dispatches on the element's ArrowType.Map it lands in MapTypeWriter.writeToListWriter, which always throws.

Common situations: Nested JSON arrays of objects being ingested into Lance; schemas ported from engines (Flink/Spark) that allow array<map<...>>; users assuming full nested-type parity in the Lance sink.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• List as map value not yet implemented
• Map as map key is not supported
• Map as map value not yet implemented
• TABLE_DATASET_WRITE_ST_ROW_EXCEPTION
• List type should be handled via FragmentConverter.writeListT

AI-assisted analysis of apache/seatunnel@cf67b549a7 (2026-09-10). Data as JSON: /api/errors/ae1db293e05e6ddd. Report an issue: GitHub.