Add cross-sharding support for discussion topics index endpoint

- Refactors related scopes for Switchman support.
- Adds an explicit shard context around the ScopedToUser call.
- Works around Rails loading with ActiveRecord descendants:
  https://github.com/rails/rails/issues/3364

fixes CNVS-24104

test plan:
- Enroll a student in two different institutions on different shards
- Merge the student if necessary
- Enroll the student in course 1 on institution 1
- Add a discussion topic for course 1 from the student.
- Access the API discussion endpoint from institution 2
- Verify that you get results.

The cross-sharded API endpoint should look something like:

institution_2.instructure.com/api/v1/courses/{global_course_id}/discussion_topics

eg

institution_2.instructure.com/api/v1/courses/10000000000001/discussion_topics

The non-sharded API endpoint should look something like:

instiution_1.instructure.com/api/v1/courses/1/discussion_topics

Both should work.

Change-Id: I54da2f5e012924c10923a8894ad8263a67fcdb06
Reviewed-on: https://gerrit.instructure.com/66599
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: Michael Hargiss <mhargiss@instructure.com>
Product-Review: Chris Wang <cwang@instructure.com>

app/controllers/discussion_topics_controller.rb

@@ -277,7 +277,11 @@ class DiscussionTopicsController < ApplicationController
               @context.active_discussion_topics.only_discussion_topics
             end
 
-    scope = DiscussionTopic::ScopedToUser.new(@context, @current_user, scope).scope
+    # Specify the shard context, because downstream we use `union` which isn't
+    # cross-shard compatible.
+    @context.shard.activate do
+      scope = DiscussionTopic::ScopedToUser.new(@context, @current_user, scope).scope
+    end
 
     scope = if params[:order_by] == 'recent_activity'
               scope.by_last_reply_at

app/models/discussion_topic.rb

@@ -19,6 +19,9 @@
 
 require 'atom'
 
+# Force loaded so that it will be in ActiveRecord::Base.descendants for switchman to use
+require_dependency 'assignment_student_visibility'
+
 class DiscussionTopic < ActiveRecord::Base
 
   include Workflow
@@ -64,6 +67,9 @@ class DiscussionTopic < ActiveRecord::Base
   has_many :child_topics, :class_name => 'DiscussionTopic', :foreign_key => :root_topic_id, :dependent => :destroy
   has_many :discussion_topic_participants, :dependent => :destroy
   has_many :discussion_entry_participants, :through => :discussion_entries
+
+  has_many :assignment_student_visibilities, :through => :assignment
+
   belongs_to :user
 
   EXPORTABLE_ATTRIBUTES = [
@@ -508,15 +514,8 @@ class DiscussionTopic < ActiveRecord::Base
   }
 
   scope :joins_assignment_student_visibilities, lambda { |user_ids, course_ids|
-    user_ids = Array.wrap(user_ids).join(',')
-    course_ids = Array.wrap(course_ids).join(',')
-    joins(sanitize_sql([<<-SQL, user_ids, course_ids]))
-      JOIN assignment_student_visibilities
-        ON (assignment_student_visibilities.assignment_id = discussion_topics.assignment_id
-          AND assignment_student_visibilities.user_id IN (%s)
-          AND assignment_student_visibilities.course_id IN (%s)
-        )
-    SQL
+    joins(:assignment_student_visibilities)
+      .where(assignment_student_visibilities: { user_id: user_ids, course_id: course_ids })
   }
 
   alias_attribute :available_from, :delayed_post_at

spec/controllers/discussion_topics_controller_spec.rb

@@ -1,4 +1,4 @@
-#
+
 # Copyright (C) 2012 Instructure, Inc.
 #
 # This file is part of Canvas.
@@ -17,6 +17,7 @@
 #
 
 require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')
+require File.expand_path(File.dirname(__FILE__) + '/../sharding_spec_helper.rb')
 
 describe DiscussionTopicsController do
   before :once do
@@ -111,6 +112,24 @@ describe DiscussionTopicsController do
       end
     end
 
+    context "cross-sharding" do
+      specs_require_sharding
+
+      it 'returns the topic across shards' do
+        @topic = @course.discussion_topics.create!(title: 'student topic', message: 'Hello', user: @student)
+        user_session(@student)
+        @shard1.activate do
+          get 'index', { format: :json, course_id: @course.id }
+          expect(assigns[:topics]).to include(@topic)
+        end
+
+        @shard2.activate do
+          get 'index', { format: :json, course_id: @course.id }
+          expect(assigns[:topics]).to include(@topic)
+        end
+      end
+    end
+
     it "should return non-graded group discussions properly" do
       @course.account.role_overrides.create!(
         role: student_role,