Fixes panic when a has-many relation contains nil

Author: brandonc

response.go

@@ -26,6 +26,8 @@ var (
 	// ErrUnexpectedType is returned when marshalling an interface; the interface
 	// had to be a pointer or a slice; otherwise this error is returned.
 	ErrUnexpectedType = errors.New("models should be a struct pointer or slice of struct pointers")
+	// ErrUnexpectedNil is returned when a slice of relation structs contains nil values
+	ErrUnexpectedNil = errors.New("slice of struct pointers cannot contain nil")
 )
 
 // MarshalPayload writes a jsonapi response for one or many records. The
@@ -498,7 +500,12 @@ func visitModelNodeRelationships(models reflect.Value, included *map[string]*Nod
 	nodes := []*Node{}
 
 	for i := 0; i < models.Len(); i++ {
-		n := models.Index(i).Interface()
+		model := models.Index(i)
+		if !model.IsValid() || model.IsNil() {
+			return nil, ErrUnexpectedNil
+		}
+
+		n := model.Interface()
 
 		node, err := visitModelNode(n, included, sideload)
 		if err != nil {

response_test.go

@@ -3,6 +3,7 @@ package jsonapi
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"reflect"
 	"sort"
@@ -40,6 +41,26 @@ func TestMarshalPayload(t *testing.T) {
 
 func TestMarshalPayloadWithNulls(t *testing.T) {
 
+func TestMarshalPayloadWithManyRelationWithNils(t *testing.T) {
+	blog := &Blog{
+		ID:    1,
+		Title: "Hello, World",
+		Posts: []*Post{
+			nil,
+			{
+				ID: 2,
+			},
+			nil,
+		},
+	}
+
+	out := bytes.NewBuffer(nil)
+	if err := MarshalPayload(out, blog); !errors.Is(err, ErrUnexpectedNil) {
+		t.Fatal("expected error but got none")
+	}
+}
+
+func TestMarshalPayloadWithNulls(t *testing.T) {
 	books := []*Book{nil, {ID: 101}, nil}
 	var jsonData map[string]interface{}