FIX: ensures we don't attempt to create a new PM on an existing topic

app/assets/javascripts/discourse/models/composer.js.es6

@@ -912,7 +912,7 @@ const Composer = RestModel.extend({
   },
 
   createPost(opts) {
-    if (this.action === CREATE_TOPIC) {
+    if (CREATE_TOPIC === this.action || PRIVATE_MESSAGE === this.action) {
       this.set("topic", null);
     }
 

config/locales/server.en.yml

@@ -339,6 +339,7 @@ en:
   pm_reached_recipients_limit: "Sorry, you can't have more than %{recipients_limit} recipients in a message."
   removed_direct_reply_full_quotes: "Automatically removed quote of whole previous post."
   secure_upload_not_allowed_in_public_topic: "Sorry, the following secure upload(s) cannot be used in a public topic: %{upload_filenames}."
+  create_pm_on_existing_topic: "Sorry, you can't create a PM on an existing topic."
 
   just_posted_that: "is too similar to what you recently posted"
   invalid_characters: "contains invalid characters"

lib/post_creator.rb

@@ -136,6 +136,14 @@ def valid?
       return false unless skip_validations? || validate_child(topic_creator)
     else
       @topic = Topic.find_by(id: @opts[:topic_id])
+
+      if @topic.present?
+        if @opts[:archetype] == Archetype.private_message
+          errors.add(:base, I18n.t(:create_pm_on_existing_topic))
+          return false
+        end
+      end
+
       unless @topic.present? && (@opts[:skip_guardian] || guardian.can_create?(Post, @topic))
         errors.add(:base, I18n.t(:topic_not_found))
         return false

spec/requests/posts_controller_spec.rb

@@ -1126,6 +1126,27 @@
         end
       end
 
+      context "when topic_id is set" do
+        fab!(:topic) { Fabricate(:topic) }
+
+        it "errors when creating a private post" do
+          user_2 = Fabricate(:user)
+
+          post "/posts.json", params: {
+            raw: 'this is the test content',
+            archetype: 'private_message',
+            title: "this is some post",
+            target_recipients: user_2.username,
+            topic_id: topic.id
+          }
+
+          expect(response.status).to eq(422)
+          expect(JSON.parse(response.body)["errors"]).to include(
+            I18n.t("create_pm_on_existing_topic")
+          )
+        end
+      end
+
       context "errors" do
         it "does not succeed" do
           post "/posts.json", params: { raw: 'test' }