[ForumPosts] Check for existence of topic & category (#524)

* [ForumPosts] Check for existence of topic & category

* [Tests] Add invalid topic category_id test

* Tweak how it is getting validated

If the category doesn't exist there's not point in doing anything further

* Move response assert before the other one

Should always be that way. If the response doesn't have the expected format
the assert will for sure fail and produce a confusing error message

---------

Co-authored-by: Earlopain <14981592+Earlopain@users.noreply.github.com>

app/models/forum_topic.rb

@@ -6,10 +6,10 @@ class ForumTopic < ApplicationRecord
   has_one :original_post, -> {order("forum_posts.id asc")}, class_name: "ForumPost", foreign_key: "topic_id", inverse_of: :topic
   has_many :subscriptions, :class_name => "ForumSubscription"
   before_validation :initialize_is_hidden, :on => :create
+  validate :category_valid
   validates :title, :creator_id, presence: true
   validates_associated :original_post
   validates_presence_of :original_post
-  validates_associated :category
   validates :title, :length => {:maximum => 250}
   validate :category_allows_creation, on: :create
   accepts_nested_attributes_for :original_post
@@ -36,6 +36,12 @@ class ForumTopic < ApplicationRecord
       category.name
     end
 
+    def category_valid
+      return if category
+      errors.add(:category, "is invalid")
+      throw :abort
+    end
+
     def category_allows_creation
       if category && !category.can_create_within?(creator)
         errors.add(:category, "does not allow new topics")

test/functional/forum_topics_controller_test.rb

@@ -102,6 +102,13 @@ class ForumTopicsControllerTest < ActionDispatch::IntegrationTest
         forum_topic = ForumTopic.last
         assert_redirected_to(forum_topic_path(forum_topic))
       end
+
+      should "fail with expected error if invalid category_id is provided" do
+        post_auth forum_topics_path, @user, params: { forum_topic: { title: "bababa", category_id: 0, original_post_attributes: { body: "xaxaxa" } }, format: :json }
+
+        assert_response :unprocessable_entity
+        assert_includes(@response.parsed_body.dig("errors", "category"), "must exist")
+      end
     end
 
     context "destroy action" do