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

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)
+	}
+}