bug: fix vist of nested struct

core/src/main/java/io/substrait/expression/FieldReference.java

@@ -40,8 +40,8 @@ public FieldReference dereferenceStruct(int index) {
   private FieldReference dereference(Type newType, ReferenceSegment nextSegment) {
     return ImmutableFieldReference.builder()
         .type(newType)
-        .addAllSegments(segments())
         .addSegments(nextSegment)
+        .addAllSegments(segments())
         .inputExpression(inputExpression())
         .build();
   }

core/src/main/java/io/substrait/expression/proto/ExpressionProtoConverter.java

@@ -325,7 +325,7 @@ public Expression visit(io.substrait.expression.Expression.MultiOrList expr)
 
   @Override
   public Expression visit(FieldReference expr) {
-    Expression.ReferenceSegment top = null;
+
     Expression.ReferenceSegment seg = null;
     for (var segment : expr.segments()) {
       Expression.ReferenceSegment.Builder protoSegment;
@@ -351,13 +351,11 @@ public Expression visit(FieldReference expr) {
         throw new IllegalArgumentException("Unhandled type: " + segment);
       }
       var builtSegment = protoSegment.build();
-      if (top == null) {
-        top = builtSegment;
-      }
       seg = builtSegment;
     }
 
-    var out = Expression.FieldReference.newBuilder().setDirectReference(top);
+    var out = Expression.FieldReference.newBuilder().setDirectReference(seg);
+
     if (expr.inputExpression().isPresent()) {
       out.setExpression(from(expr.inputExpression().get()));
     } else if (expr.outerReferenceStepsOut().isPresent()) {

core/src/main/java/io/substrait/expression/proto/ProtoExpressionConverter.java

@@ -11,6 +11,7 @@
 import io.substrait.type.Type;
 import io.substrait.type.proto.ProtoTypeConverter;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Objects;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
@@ -67,7 +68,7 @@ public FieldReference from(io.substrait.proto.Expression.FieldReference referenc
                     "Unhandled type: " + segment.getReferenceTypeCase());
               });
         }
-
+        Collections.reverse(segments);
         var fieldReference =
             switch (reference.getRootTypeCase()) {
               case EXPRESSION -> FieldReference.ofExpression(

isthmus/src/main/java/io/substrait/isthmus/expression/RexExpressionConverter.java

@@ -14,9 +14,11 @@
 import java.util.stream.Collectors;
 import org.apache.calcite.rel.RelNode;
 import org.apache.calcite.rex.*;
+import org.apache.calcite.sql.SqlKind;
 import org.apache.calcite.sql.fun.SqlStdOperatorTable;
 
 public class RexExpressionConverter implements RexVisitor<Expression> {
+
   static final org.slf4j.Logger logger =
       org.slf4j.LoggerFactory.getLogger(RexExpressionConverter.class);
 
@@ -119,16 +121,28 @@ public Expression visitRangeRef(RexRangeRef rangeRef) {
 
   @Override
   public Expression visitFieldAccess(RexFieldAccess fieldAccess) {
-    if (fieldAccess.getReferenceExpr() instanceof RexCorrelVariable) {
-      int stepsOut = relVisitor.getFieldAccessDepth(fieldAccess);
-
-      return FieldReference.newRootStructOuterReference(
-          fieldAccess.getField().getIndex(),
-          typeConverter.toSubstrait(fieldAccess.getType()),
-          stepsOut);
+    SqlKind kind = fieldAccess.getReferenceExpr().getKind();
+    switch (kind) {
+      case CORREL_VARIABLE -> {
+        int stepsOut = relVisitor.getFieldAccessDepth(fieldAccess);
+
+        return FieldReference.newRootStructOuterReference(
+            fieldAccess.getField().getIndex(),
+            typeConverter.toSubstrait(fieldAccess.getType()),
+            stepsOut);
+      }
+      case ITEM, INPUT_REF, FIELD_ACCESS -> {
+        Expression expression = fieldAccess.getReferenceExpr().accept(this);
+        if (expression instanceof FieldReference) {
+          FieldReference nestedReference = (FieldReference) expression;
+          return nestedReference.dereferenceStruct(fieldAccess.getField().getIndex());
+        } else {
+          return FieldReference.newStructReference(fieldAccess.getField().getIndex(), expression);
+        }
+      }
+      default -> throw new UnsupportedOperationException(
+          String.format("RexFieldAccess for SqlKind %s not supported", kind));
     }
-    throw new UnsupportedOperationException(
-        "RexFieldAccess for other than RexCorrelVariable not supported");
   }
 
   @Override