Serialize empty data as empty array

document.go

@@ -12,7 +12,7 @@ Refer to the JSON API Specification for a full descriptor
 of each attribute: http://jsonapi.org/format/#document-structure
 */
 type Document struct {
-	Data     List        `json:"data,omitempty"`
+	Data     List        `json:"data"`
 	Errors   ErrorList   `json:"errors,omitempty"`
 	Links    *Link       `json:"links,omitempty"`
 	Included []*Object   `json:"included,omitempty"`
@@ -89,10 +89,10 @@ func (d *Document) Validate(r *http.Request, response bool) *Error {
 		return nil
 	}
 
-	if !d.HasErrors() && !d.HasData() {
+	if !d.HasErrors() && d.Data == nil {
 		return ISE("Both `errors` and `data` cannot be blank for a JSON response")
 	}
-	if d.HasErrors() && d.HasData() {
+	if d.HasErrors() && d.Data != nil {
 		return ISE("Both `errors` and `data` cannot be set for a JSON response")
 	}
 	if !d.HasData() && d.Included != nil {

list.go

@@ -55,15 +55,23 @@ func (list *List) UnmarshalJSON(rawData []byte) error {
 MarshalJSON returns a top level object for the "data" attribute if a single object. In
 all other cases returns a JSON encoded list for "data".
 */
-func (list List) MarshalJSON() ([]byte, error) {
+func (list *List) MarshalJSON() ([]byte, error) {
 	// avoid stack overflow by using this subtype for marshaling
 	type MarshalList List
-	marshalList := MarshalList(list)
-	count := len(marshalList)
+	marshalList := (*MarshalList)(list)
+	isnil := marshalList == nil
+	count := 0
+	if !isnil {
+		count = len(*marshalList)
+	}
 
 	switch {
+	case isnil:
+		return nil, nil
+	case count == 0:
+		return []byte("[]"), nil
 	case count == 1:
-		return json.Marshal(marshalList[0])
+		return json.Marshal((*marshalList)[0])
 	default:
 		return json.Marshal(marshalList)
 	}