Preserve nested values on parent path conflicts

cmd/dataxl/main_test.go

@@ -138,12 +138,24 @@ func TestParseCellConservativeInference(t *testing.T) {
 }
 
 func TestSetPathDoesNotOverwriteConflictingShape(t *testing.T) {
+	t.Run("parent scalar before child", func(t *testing.T) {
 		record := map[string]any{}
 		setPath(record, "user", "Alice")
 		setPath(record, "user.name", "Bob")
 		if got := record["user"]; got != "Alice" {
 			t.Fatalf("user = %#v, want original scalar", got)
 		}
+	})
+
+	t.Run("child before parent scalar", func(t *testing.T) {
+		record := map[string]any{}
+		setPath(record, "user.name", "Bob")
+		setPath(record, "user", "Alice")
+		want := map[string]any{"name": "Bob"}
+		if got := record["user"]; !reflect.DeepEqual(got, want) {
+			t.Fatalf("user = %#v, want original nested value %#v", got, want)
+		}
+	})
 }
 
 func TestSetPathRestoresNestedArrays(t *testing.T) {

cmd/dataxl/path.go

@@ -93,7 +93,12 @@ func setPath(root map[string]any, path string, value any) {
 			return
 		}
 		if last {
+			// Preserve the value established by an earlier column. This also
+			// protects a nested map/slice when a later parent scalar conflicts
+			// with it (for example, user.name followed by user).
+			if _, exists := m[token.key]; !exists {
 				m[token.key] = value
+			}
 			return
 		}
 		if _, exists := m[token.key]; !exists {