FIX: Ensure first post is loaded before trying to bookmark topic (#9382)

Load first post if not loaded when trying to bookmark topic

* if the first post was not loaded we could not bookmark it because
  now we call the toggleBookmarkReminder function on an actual post
  to open the modal window
* add a firstPost function to the topic model to get the first post
  from the stream if it is loaded and if not do a GET request to load
  it
* use the firstPost promise when bookmarking topic

app/assets/javascripts/discourse/controllers/topic.js

@@ -453,22 +453,9 @@ export default Controller.extend(bufferedProperty("model"), {
     },
 
     editFirstPost() {
-      const postStream = this.get("model.postStream");
+      this.model
-      let firstPost = postStream.get("posts.firstObject");
+        .firstPost()
-
+        .then(firstPost => this.send("editPost", firstPost));
-      if (firstPost.get("post_number") !== 1) {
-        const postId = postStream.findPostIdForPostNumber(1);
-        // try loading from identity map first
-        firstPost = postStream.findLoadedPost(postId);
-        if (firstPost === undefined) {
-          return this.get("model.postStream")
-            .loadPost(postId)
-            .then(post => {
-              this.send("editPost", post);
-            });
-        }
-      }
-      this.send("editPost", firstPost);
     },
 
     // Post related methods

app/assets/javascripts/discourse/models/topic.js

@@ -1,4 +1,4 @@
-import EmberObject, { get } from "@ember/object";
+import EmberObject from "@ember/object";
 import { not, notEmpty, equal, and, or } from "@ember/object/computed";
 import { ajax } from "discourse/lib/ajax";
 import { flushMap } from "discourse/models/store";
@@ -407,18 +407,34 @@ const Topic = RestModel.extend({
     }
   },
 
+  firstPost() {
+    const postStream = this.postStream;
+    let firstPost = postStream.get("posts.firstObject");
+
+    if (firstPost.post_number === 1) {
+      return Promise.resolve(firstPost);
+    }
+
+    const postId = postStream.findPostIdForPostNumber(1);
+
+    // try loading from identity map first
+    firstPost = postStream.findLoadedPost(postId);
+    if (firstPost) {
+      return Promise.resolve(firstPost);
+    }
+
+    return this.postStream.loadPost(postId);
+  },
+
   toggleBookmark() {
     if (this.bookmarking) {
       return Promise.resolve();
     }
     this.set("bookmarking", true);
-
-    const stream = this.postStream;
-    const posts = get(stream, "posts");
-    const firstPost =
-      posts && posts[0] && posts[0].get("post_number") === 1 && posts[0];
     const bookmark = !this.bookmarked;
+    let posts = this.postStream.posts;
 
+    return this.firstPost().then(firstPost => {
       const toggleBookmarkOnServer = () => {
         if (bookmark) {
           if (this.siteSettings.enable_bookmarks_with_reminders) {
@@ -442,13 +458,13 @@ const Topic = RestModel.extend({
               if (posts) {
                 const updated = [];
                 posts.forEach(post => {
-                if (post.get("bookmarked")) {
+                  if (post.bookmarked) {
                     post.set("bookmarked", false);
                     updated.push(post.id);
                   }
                   if (
                     this.siteSettings.enable_bookmarks_with_reminders &&
-                  post.get("bookmarked_with_reminder")
+                    post.bookmarked_with_reminder
                   ) {
                     post.set("bookmarked_with_reminder", false);
                     updated.push(post.id);
@@ -466,7 +482,7 @@ const Topic = RestModel.extend({
       if (!bookmark && posts) {
         posts.forEach(
           post =>
-          (post.get("bookmarked") || post.get("bookmarked_with_reminder")) &&
+            (post.bookmarked || post.bookmarked_with_reminder) &&
             unbookmarkedPosts.push(post)
         );
       }
@@ -484,6 +500,7 @@ const Topic = RestModel.extend({
           toggleBookmarkOnServer().then(resolve);
         }
       });
+    });
   },
 
   createGroupInvite(group) {