Add optional subject param to tx API. Closes #2333.

docs/docs/content/apis/transactional.md

@@ -20,6 +20,7 @@ Allows sending transactional messages to one or more subscribers via a preconfig
 | subscriber_ids    | number\[\]  |          | Multiple subscriber IDs as an alternative to `subscriber_id`.              |
 | template_id       | number    | Yes      | ID of the transactional template to be used for the message.               |
 | from_email        | string    |          | Optional sender email.                                                     |
+| subject           | string    |          | Optional subject. If empty, the subject defined on the template is used    |
 | data              | JSON      |          | Optional nested JSON map. Available in the template as `{{ .Tx.Data.* }}`. |
 | headers           | JSON\[\]    |          | Optional array of email headers.                                           |
 | messenger         | string    |          | Messenger to send the message. Default is `email`.                         |

models/models.go

@@ -387,11 +387,11 @@ type TxMessage struct {
 	Headers     Headers        `json:"headers"`
 	ContentType string         `json:"content_type"`
 	Messenger   string         `json:"messenger"`
+	Subject     string         `json:"subject"`
 
 	// File attachments added from multi-part form data.
 	Attachments []Attachment `json:"-"`
 
-	Subject    string             `json:"-"`
 	Body       []byte             `json:"-"`
 	Tpl        *template.Template `json:"-"`
 	SubjectTpl *txttpl.Template   `json:"-"`
@@ -654,15 +654,35 @@ func (m *TxMessage) Render(sub Subscriber, tpl *Template) error {
 	copy(m.Body, b.Bytes())
 	b.Reset()
 
+	// Was a subject provided in the message?
+	var (
+		subjTpl *txttpl.Template
+		subject = m.Subject
+	)
+	if subject != "" {
+		if strings.Contains(m.Subject, "{{") {
+			// If the subject has a template string, render that.
+			s, err := txttpl.New(BaseTpl).Funcs(txttpl.FuncMap(nil)).Parse(m.Subject)
+			if err != nil {
+				return fmt.Errorf("error compiling subject: %v", err)
+			}
+			subjTpl = s
+		}
+	} else {
+		// Use the subject from the template.
+		subject = tpl.Subject
+		subjTpl = tpl.SubjectTpl
+	}
+
 	// If the subject is also a template, render that.
-	if tpl.SubjectTpl != nil {
+	if subjTpl != nil {
-		if err := tpl.SubjectTpl.ExecuteTemplate(&b, BaseTpl, data); err != nil {
+		if err := subjTpl.ExecuteTemplate(&b, BaseTpl, data); err != nil {
 			return err
 		}
 		m.Subject = b.String()
 		b.Reset()
 	} else {
-		m.Subject = tpl.Subject
+		m.Subject = subject
 	}
 
 	return nil