When creating a topic, don't select a category by default when allow_uncategorized_topics is false. Also, added category validation on the server to enforce allow_uncategorized_topics.

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

@@ -339,10 +339,6 @@ Discourse.Composer = Discourse.Model.extend({
       reply: opts.reply || this.get("reply") || ""
     });
 
-    if (!this.get('categoryId') && !Discourse.SiteSettings.allow_uncategorized_topics && Discourse.Category.list().length > 0) {
-      this.set('categoryId', Discourse.Category.list()[0].get('id'));
-    }
-
     if (opts.postId) {
       this.set('loading', true);
       Discourse.Post.load(opts.postId).then(function(result) {

app/assets/javascripts/discourse/views/category_chooser_view.js

@@ -21,7 +21,11 @@ Discourse.CategoryChooserView = Discourse.ComboboxView.extend({
   },
 
   none: function() {
-    if (Discourse.SiteSettings.allow_uncategorized_topics || this.get('showUncategorized')) return 'category.none';
+    if (!Discourse.SiteSettings.allow_uncategorized_topics) {
+      return 'category.choose';
+    } else if (Discourse.SiteSettings.allow_uncategorized_topics || this.get('showUncategorized')) {
+      return 'category.none';
+    }
   }.property('showUncategorized'),
 
   template: function(text, templateData) {

app/assets/stylesheets/desktop/compose.scss

@@ -335,7 +335,7 @@
   .category-input .popup-tip {
     width: 240px;
     left: 432px;
-    top: -7px;
+    top: -19px;
   }
 }
 

app/controllers/topics_controller.rb

@@ -108,7 +108,7 @@ class TopicsController < ApplicationController
     success = false
     Topic.transaction do
       success = topic.save
-      topic.change_category(params[:category]) if success
+      success = topic.change_category(params[:category]) if success
     end
 
     # this is used to return the title to the client as it may have been

app/models/topic.rb

@@ -56,6 +56,12 @@ class Topic < ActiveRecord::Base
                                         :case_sensitive => false,
                                         :collection => Proc.new{ Topic.listable_topics } }
 
+  # The allow_uncategorized_topics site setting can be changed at any time, so there may be
+  # existing topics with nil category. We'll allow that, but when someone tries to make a new
+  # topic or change a topic's category, perform validation.
+  attr_accessor :do_category_validation
+  validates :category_id, :presence => { :if => Proc.new { @do_category_validation && !SiteSetting.allow_uncategorized_topics } }
+
   before_validation do
     self.sanitize_title
     self.title = TextCleaner.clean_title(TextSentinel.title_sentinel(title).text) if errors[:title].empty?
@@ -135,6 +141,7 @@ class Topic < ActiveRecord::Base
   before_create do
     self.bumped_at ||= Time.now
     self.last_post_user_id ||= user_id
+    self.do_category_validation = true
     if !@ignore_category_auto_close and self.category and self.category.auto_close_days and self.auto_close_at.nil?
       set_auto_close(self.category.auto_close_days)
     end
@@ -316,8 +323,8 @@ class Topic < ActiveRecord::Base
 
   def changed_to_category(cat)
 
-    return if cat.blank?
-    return if Category.where(topic_id: id).first.present?
+    return true if cat.blank?
+    return true if Category.where(topic_id: id).first.present?
 
     Topic.transaction do
       old_category = category
@@ -327,13 +334,16 @@ class Topic < ActiveRecord::Base
       end
 
       self.category_id = cat.id
-      save
-
+      if save
         CategoryFeaturedTopic.feature_topics_for(old_category)
         Category.where(id: cat.id).update_all 'topic_count = topic_count + 1'
         CategoryFeaturedTopic.feature_topics_for(cat) unless old_category.try(:id) == cat.try(:id)
+      else
+        return false
       end
     end
+    true
+  end
 
   def add_moderator_post(user, text, opts={})
     new_post = nil
@@ -362,6 +372,8 @@ class Topic < ActiveRecord::Base
 
   # Changes the category to a new name
   def change_category(name)
+    self.do_category_validation = true
+
     # If the category name is blank, reset the attribute
     if name.blank?
       if category_id.present?
@@ -369,12 +381,11 @@ class Topic < ActiveRecord::Base
         Category.where(id: category_id).update_all 'topic_count = topic_count - 1'
       end
       self.category_id = nil
-      save
-      return
+      return save
     end
 
     cat = Category.where(name: name).first
-    return if cat == category
+    return true if cat == category
     changed_to_category(cat)
   end
 

config/locales/client.en.yml

@@ -925,6 +925,7 @@ en:
     category:
       can: 'can… '
       none: '(no category)'
+      choose: 'Select a category…'
       edit: 'edit'
       edit_long: "Edit Category"
       edit_uncategorized: "Edit Uncategorized"

spec/controllers/topics_controller_spec.rb

@@ -637,7 +637,7 @@ describe TopicsController do
         end
 
         it 'triggers a change of category' do
-          Topic.any_instance.expects(:change_category).with('incredible')
+          Topic.any_instance.expects(:change_category).with('incredible').returns(true)
           xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category: 'incredible'
         end
 
@@ -646,6 +646,24 @@ describe TopicsController do
           expect(response).not_to be_success
         end
 
+        it "returns errors with invalid categories" do
+          Topic.any_instance.expects(:change_category).returns(false)
+          xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category: ''
+          expect(response).not_to be_success
+        end
+
+        context "allow_uncategorized_topics is false" do
+          before do
+            SiteSetting.stubs(:allow_uncategorized_topics).returns(false)
+          end
+
+          it "can add a category to an uncategorized topic" do
+            Topic.any_instance.expects(:change_category).with('incredible').returns(true)
+            xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category: 'incredible'
+            response.should be_success
+          end
+        end
+
       end
     end
   end

spec/models/topic_spec.rb

@@ -175,6 +175,38 @@ describe Topic do
 
   end
 
+  context 'category validation' do
+    context 'allow_uncategorized_topics is false' do
+      before do
+        SiteSetting.stubs(:allow_uncategorized_topics).returns(false)
+      end
+
+      it "does not allow nil category" do
+        topic = Fabricate(:topic, category: nil)
+        topic.should_not be_valid
+        topic.errors[:category_id].should be_present
+      end
+
+      it 'passes for topics with a category' do
+        Fabricate.build(:topic, category: Fabricate(:category)).should be_valid
+      end
+    end
+
+    context 'allow_uncategorized_topics is true' do
+      before do
+        SiteSetting.stubs(:allow_uncategorized_topics).returns(true)
+      end
+
+      it "passes for topics with nil category" do
+        Fabricate.build(:topic, category: nil).should be_valid
+      end
+
+      it 'passes for topics with a category' do
+        Fabricate.build(:topic, category: Fabricate(:category)).should be_valid
+      end
+    end
+  end
+
 
   context 'similar_to' do
 
@@ -825,7 +857,18 @@ describe Topic do
         it "should lower the original category's topic count" do
           @category.topic_count.should == 0
         end
+      end
 
+      context 'when allow_uncategorized_topics is false' do
+        before do
+          SiteSetting.stubs(:allow_uncategorized_topics).returns(false)
+        end
+
+        let!(:topic) { Fabricate(:topic, category: Fabricate(:category)) }
+
+        it 'returns false' do
+          topic.change_category('').should eq(false) # don't use "be_false" here because it would also match nil
+        end
       end
 
       describe 'when the category exists' do
@@ -838,7 +881,6 @@ describe Topic do
           @topic.category_id.should be_blank
           @category.topic_count.should == 0
         end
-
       end
 
     end

test/javascripts/models/composer_test.js

@@ -186,5 +186,5 @@ test('initial category when uncategorized is allowed', function() {
 test('initial category when uncategorized is not allowed', function() {
   Discourse.SiteSettings.allow_uncategorized_topics = false;
   var composer = Discourse.Composer.open({action: 'createTopic', draftKey: 'asfd', draftSequence: 1});
-  ok(composer.get('categoryId') !== undefined, "Not uncategorized by default");
+  ok(composer.get('categoryId') === undefined, "Uncategorized by default. Must choose a category.");
 });