FEATURE: show a reason why a category can't be deleted

app/assets/javascripts/discourse/templates/modal/edit-category.js.handlebars

@@ -18,6 +18,10 @@
     <button class='btn btn-primary' {{bind-attr disabled="disabled"}} {{action saveCategory}}>{{buttonTitle}}</button>
     {{#if can_delete}}
       <button class='btn btn-danger pull-right' {{bind-attr disabled="deleteDisabled"}} {{action deleteCategory}}><i class="fa fa-trash-o"></i>{{deleteButtonTitle}}</button>
+    {{else}}
+      <div class="cannot_delete_reason">
+        {{{cannot_delete_reason}}}
+      </div>
     {{/if}}
   </div>
 </div>

app/assets/stylesheets/desktop/modal.scss

@@ -127,6 +127,13 @@ animation: modal .25s;
       }
     }
   }
+
+  .cannot_delete_reason {
+    float: right;
+    text-align: right;
+    max-width: 380px;
+    color: scale-color($primary, $lightness: 60%);
+  }
 }
 
 .topic-bulk-actions-modal {

app/serializers/category_serializer.rb

@@ -8,6 +8,7 @@ class CategorySerializer < BasicCategorySerializer
              :email_in,
              :email_in_allow_strangers,
              :can_delete,
+             :cannot_delete_reason,
              :allow_badges
 
   def group_permissions
@@ -38,6 +39,14 @@ class CategorySerializer < BasicCategorySerializer
     scope && scope.can_delete?(object)
   end
 
+  def cannot_delete_reason
+    scope && scope.cannot_delete_category_reason(object)
+  end
+
+  def include_cannot_delete_reason
+    !include_can_delete? && scope && scope.can_edit?(object)
+  end
+
   def include_email_in?
     scope && scope.can_edit?(object)
   end

config/locales/server.en.yml

@@ -268,6 +268,13 @@ en:
       uncategorized_parent: "Uncategorized can't have a parent category"
       self_parent: "A subcategory's parent cannot be itself"
       depth: "You can't nest a subcategory under another"
+    cannot_delete:
+      uncategorized: "Can't delete Uncategorized"
+      has_subcategories: "Can't delete this category because it has sub-categories."
+      topic_exists:
+        one: "Can't delete this category because is has 1 topic. Oldest topic is %{topic_link}."
+        other: "Can't delete this category because it has %{count} topics. Oldest topic is %{topic_link}."
+      topic_exists_no_oldest: "Can't delete this category because topic count is #{category.topic_count}."
   trust_levels:
     newuser:
       title: "new user"

lib/guardian/category_guardian.rb

@@ -27,6 +27,23 @@ module CategoryGuardian
     !category.has_children?
   end
 
+  def cannot_delete_category_reason(category)
+    return I18n.t('category.cannot_delete.uncategorized') if category.uncategorized?
+    return I18n.t('category.cannot_delete.has_subcategories') if category.has_children?
+
+    if category.topic_count != 0
+      oldest_topic = category.topics.where.not(id: category.topic_id).order('created_at ASC').limit(1).first
+      if oldest_topic
+        return I18n.t('category.cannot_delete.topic_exists', {count: category.topic_count, topic_link: "<a href=\"#{oldest_topic.url}\">#{oldest_topic.title}</a>"})
+      else
+        # This is a weird case, probably indicating a bug.
+        return I18n.t('category.cannot_delete.topic_exists_no_oldest', {count: category.topic_count})
+      end
+    end
+
+    nil
+  end
+
   def can_see_category?(category)
     not(category.read_restricted) || secure_category_ids.include?(category.id)
   end