columnar: fix handling of json:"-" fields

3 files changed, 27 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e41b7c4..96f65a2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@ SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
 SPDX-License-Identifier: Apache-2.0
 -->
 
+# v1.9.1 (TBD)
+
+Changes:
+
+- Fix columnar choking on structs with `json:"-"` fields.
+
 # v1.9.0 (2026-06-04)
 
 Changes:
diff --git a/columnar/columnar.go b/columnar/columnar.go
index bb549f8..5e339ab 100644
--- a/columnar/columnar.go
+++ b/columnar/columnar.go
@@ -59,7 +59,7 @@ type List[T any] []T
 func foreachRelevantField(t reflect.Type, action func(f reflect.StructField)) {
 	for idx := range t.NumField() {
 		f := t.Field(idx)
-		if f.PkgPath == "" {
+		if f.PkgPath == "" && f.Tag.Get("json") != "-" {
 			action(f)
 		}
 	}
diff --git a/columnar/columnar_test.go b/columnar/columnar_test.go
index 17dc964..70e27c4 100644
--- a/columnar/columnar_test.go
+++ b/columnar/columnar_test.go
@@ -116,3 +116,23 @@ func TestJSONUnmarshalFromInconsistentLengths(t *testing.T) {
 	err := json.Unmarshal([]byte(`{"Foo":[1,2],"Bar":[3,4,5]}`), PointerTo(columnar.List[Record]{}))
 	AssertEqual(t, err.Error(), `cannot unmarshal from columns with inconsistent lengths [2 3]`)
 }
+
+func TestIgnoredFields(t *testing.T) {
+	type Record struct {
+		Foo   int `json:"foo"`
+		Bonus int `json:"-"`
+	}
+
+	// There used to be a bug where columnar got confused that the `Bonus` field
+	// ends up as a zero-length array after unmarshaling.
+	testSuccessfulJSONRoundtrip(t,
+		[]Record{
+			{Foo: 1, Bonus: 0},
+			{Foo: 2, Bonus: 0},
+			{Foo: 3, Bonus: 0},
+		},
+		jsonmatch.Object{
+			"foo": jsonmatch.Array{1, 2, 3},
+		},
+	)
+}