allow cross shard enrollments to view peer reviews

fixes VICE-769
flag=none

test plan:
  - grab a snack, you might be here awhile
  - create two accounts that are trusted
    - you'll need mra configured on your installation
    - once you have the two accounts created, create a trust:
      > Account.create_web_of_trust([account1, account2])
    - add to the account your course will live in a new auth provider
      - visit /accounts/:id/authentication_providers
      - select 'trusted canvas instance' in the identity provider dropdown
      - select the other account as the provider
        - if this fails, ensure both accounts have populated domains
          > account.account_domains.create(host: 'foo.bar') # if not
  - make sure to eat your snack, we're only halfway done
  - in one of the accounts, create a course with a graded discussion topic
    - enroll two users from the second account
    - as both users, leave a reply to the discussion
    - enable the peer review option for grading
    - assign the two users as peer reviewers for each other
      - this can be done at /courses/:id/assignments/:id/peer_reviews
    - as both of the users, visit the discussion once again
      - you should see that each user is assigned as a peer reviewer
  - if you really want to get fancy, test the same setup, but with
    users on the same shard as the course
    - or re-read the tests and see we should be good \o/

qa risk: low

Change-Id: Iebf08ffdaed9c268b22728f53042a377df9608ab
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/247519
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Matthew Lemon <mlemon@instructure.com>
QA-Review: Matthew Lemon <mlemon@instructure.com>
Product-Review: Matthew Lemon <mlemon@instructure.com>

app/presenters/discussion_topic_presenter.rb

@@ -66,7 +66,7 @@ class DiscussionTopicPresenter
   end
 
   def peer_reviews_for(user)
-    reviews = user.assigned_submission_assessments.for_assignment(assignment.id).to_a
+    reviews = user.assigned_submission_assessments.shard(assignment.shard).for_assignment(assignment.id).to_a
     if reviews.any?
       valid_student_ids = assignment.context.participating_students.where(:id => reviews.map(&:user_id)).pluck(:id).to_set
       reviews = reviews.select{|r| valid_student_ids.include?(r.user_id)}

spec/presenters/discussion_topic_presenter_spec.rb

@@ -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 DiscussionTopicPresenter do
   let (:topic)      { DiscussionTopic.new(:title => 'Test Topic', :assignment => assignment) }
@@ -191,4 +192,39 @@ describe DiscussionTopicPresenter do
     end
   end
 
+  describe "#peer_reviews_for" do
+    let(:user2) { user_model }
+    let(:user3) { user_model }
+
+    before do
+      topic.context = course
+      assignment.context = course
+      assignment.peer_reviews = true
+      assignment.save!
+      [user, user2, user3].map do |u|
+        course.enroll_student(u, {enrollment_state: 'active'})
+      end
+      assignment.assign_peer_review(user, user2)
+    end
+
+    it "returns the assigned peer reviews" do
+      expect(
+        presenter.peer_reviews_for(user).map(&:user_id)
+      ).to match_array [user2.id]
+    end
+
+    context "for cross-shard enrollments" do
+      specs_require_sharding
+
+      let(:user) { @shard2.activate { user_model } }
+      let(:user2) { @shard2.activate { user_model } }
+
+      it "returns the assigned peer reviews" do
+        expect(
+          presenter.peer_reviews_for(user).map(&:user_id)
+        ).to match_array [user2.id]
+      end
+    end
+  end
+
 end