fix(cabana): show to-many relation list columns joined with commas

docs/backend/relation-manager.md

@@ -236,4 +236,4 @@ A relation's `messages` block overrides the relation manager's copy, each key a
 
 ## Relations in lists
 
-A list column can show a related value with `relation` and `select` in `columns.yaml`; see [Lists and filters](lists-and-filters.md). A controller that maps a relation column to a physical column itself implements `pact.ListRelationColumnMapper`.
+A list column can show a related value with `relation` and `select` in `columns.yaml`; see [Lists and filters](lists-and-filters.md). On a to-many relation the column joins the selected value of every related record with `, `, as WinterCMS does, and stays empty when there are none. A controller that maps a relation column to a physical column itself implements `pact.ListRelationColumnMapper`.

modules/cabana/model_fields_test.go

@@ -84,3 +84,41 @@ func TestFieldByColumnTagBeatsGoName(t *testing.T) {
 		t.Fatalf("a tagged field matched by its Go name: %v", got)
 	}
 }
+
+type listTag struct {
+	ID   uint    `gorm:"primarykey"`
+	Name *string `gorm:"column:name"`
+}
+
+type listTagged struct {
+	ID       uint       `gorm:"primarykey"`
+	Tags     []listTag  `gorm:"many2many:tagged_tags"`
+	Pointers []*listTag `gorm:"many2many:tagged_pointer_tags"`
+}
+
+// TestProjectRowJoinsToManyRelation pins a to-many relation column (relation:
+// tags, select: name): the list shows every related record's selected value
+// joined with ", " like Winter, and leaves the key unset when there are none.
+func TestProjectRowJoinsToManyRelation(t *testing.T) {
+	news, guides := "News", "Guides"
+	row := &listTagged{
+		ID:       3,
+		Tags:     []listTag{{ID: 1, Name: &news}, {ID: 2, Name: &guides}, {ID: 4}},
+		Pointers: []*listTag{nil, {ID: 2, Name: &guides}},
+	}
+	cols := []ListColumn{
+		{Key: "tags", Relation: "tags", Select: "name"},
+		{Key: "pointers", Relation: "pointers", Select: "name"},
+	}
+	projected := projectRow(row, nil, cols)
+	if projected["tags"] != "News, Guides" {
+		t.Fatalf("tags = %#v, want %q", projected["tags"], "News, Guides")
+	}
+	if projected["pointers"] != "Guides" {
+		t.Fatalf("pointers = %#v, want %q", projected["pointers"], "Guides")
+	}
+	empty := projectRow(&listTagged{ID: 5}, nil, cols)
+	if _, ok := empty["tags"]; ok {
+		t.Fatalf("empty relation projected %#v, want the key unset", empty)
+	}
+}

modules/cabana/query.go

@@ -580,16 +580,48 @@ func relatedSelect(row reflect.Value, controller pact.AdminController, col ListC
 		}
 		field = field.Elem()
 	}
+	column := listRelationColumn(controller, col.Relation, col.Select)
+	if field.Kind() == reflect.Slice {
+		return relatedSelectMany(field, column)
+	}
 	if field.Kind() != reflect.Struct {
 		return nil, false
 	}
-	selected := fieldByColumn(field, listRelationColumn(controller, col.Relation, col.Select))
+	selected := fieldByColumn(field, column)
 	if !selected.IsValid() || !selected.CanInterface() {
 		return nil, false
 	}
 	return selected.Interface(), true
 }
 
+// relatedSelectMany joins the selected column of every related record with
+// ", ", as Winter's list does for a to-many relation column. No related
+// records leave the key unset, so the list shows its empty placeholder.
+func relatedSelectMany(records reflect.Value, column string) (any, bool) {
+	values := make([]string, 0, records.Len())
+	for i := 0; i < records.Len(); i++ {
+		record := records.Index(i)
+		for record.Kind() == reflect.Pointer && !record.IsNil() {
+			record = record.Elem()
+		}
+		if record.Kind() != reflect.Struct {
+			continue
+		}
+		selected := fieldByColumn(record, column)
+		for selected.IsValid() && selected.Kind() == reflect.Pointer && !selected.IsNil() {
+			selected = selected.Elem()
+		}
+		if !selected.IsValid() || !selected.CanInterface() || selected.Kind() == reflect.Pointer {
+			continue
+		}
+		values = append(values, fmt.Sprint(selected.Interface()))
+	}
+	if len(values) == 0 {
+		return nil, false
+	}
+	return strings.Join(values, ", "), true
+}
+
 func listRelationColumn(controller pact.AdminController, relation, column string) string {
 	mapper, ok := controller.(pact.ListRelationColumnMapper)
 	if !ok || mapper == nil {