FIX: Publish group's categories to user when adding user to group.

https://meta.discourse.org/t/joining-a-group-doesnt-take-effect-unless-the-browser-is-hard-refreshed/57357/11?u=tgxworld

app/models/group.rb

@@ -371,6 +371,11 @@ class Group < ActiveRecord::Base
 
   def add(user)
     self.users.push(user) unless self.users.include?(user)
+
+    MessageBus.publish('/categories', {
+      categories: ActiveModel::ArraySerializer.new(self.categories).as_json
+    }, user_ids: [user.id])
+
     self
   end
 

spec/models/group_spec.rb

@@ -3,6 +3,7 @@ require 'rails_helper'
 describe Group do
   let(:admin) { Fabricate(:admin) }
   let(:user) { Fabricate(:user) }
+  let(:group) { Fabricate(:group) }
 
   describe '#builtin' do
     context "verify enum sequence" do
@@ -450,4 +451,20 @@ describe Group do
       end
     end
   end
+
+  describe '#add' do
+    context 'when adding a user into a public group' do
+      let(:category) { Fabricate(:category) }
+
+      it "should publish the group's categories to the client" do
+        group.update!(public: true, categories: [category])
+
+        message = MessageBus.track_publish { group.add(user) }.first
+
+        expect(message.data[:categories].count).to eq(1)
+        expect(message.data[:categories].first[:id]).to eq(category.id)
+        expect(message.user_ids).to eq([user.id])
+      end
+    end
+  end
 end