fix grade calculator to handle deleted enrollments

The grade calculator was throwing an error when computing scores for
a grading period for users with deleted enrollments. It will no longer
throw an error in that scenario.

closes CNVS-34643

1. Set up a course that is using grading periods with at least two
   active students.
2. Open a rails console.
3. Find the course.

   course = Course.find(2)

4. Find an active student in the course.

   student = course.students.active.first

5. Destroy all the student's enrollments.

   student.enrollments.destroy_all

6. Get a grading period ID for a grading period being used by the
   course.

   grading_period_id = GradingPeriod.for(course).first.id

7. Run the grade calculator for the student in the grading period and
   verify no errors are thrown.

   GradeCalculator.recompute_final_score(
     student.id,
     course.id,
     grading_period_id: grading_period_id
   )

Change-Id: If576c62fed75666c259d4753e8e7e7aeb1a135c9
Reviewed-on: https://gerrit.instructure.com/100961
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Reviewed-by: Neil Gupta <ngupta@instructure.com>
Reviewed-by: Keith T. Garner <kgarner@instructure.com>
Tested-by: Jenkins
Reviewed-by: Derek Bender <djbender@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: KC Naegle <knaegle@instructure.com>
Product-Review: Keith T. Garner <kgarner@instructure.com>

lib/grade_calculator.rb

@@ -261,7 +261,7 @@ class GradeCalculator
     visible_assignments = visible_assignments.select{|a| assignment_ids_visible_to_user(user_id).include?(a.id)}
 
     if @grading_period
-      user = @course.users.find(user_id)
+      user = User.find(user_id)
       visible_assignments = @grading_period.assignments_for_student(visible_assignments, user)
     end
     assignments_by_group_id = visible_assignments.group_by(&:assignment_group_id)

spec/lib/grade_calculator_spec.rb

@@ -33,6 +33,21 @@ describe GradeCalculator do
       expect(@user.enrollments.first.computed_final_score).to eql(25.0)
     end
 
+    it "can compute scores for users with deleted enrollments when grading periods are used" do
+      @course.root_account.enable_feature!(:multiple_grading_periods)
+      grading_period_set = @course.root_account.grading_period_groups.create!
+      grading_period_set.enrollment_terms << @course.enrollment_term
+      period = grading_period_set.grading_periods.create!(
+        title: "A Grading Period",
+        start_date: 10.days.ago,
+        end_date: 10.days.from_now
+      )
+      @user.enrollments.first.destroy
+      expect {
+        GradeCalculator.recompute_final_score(@user.id, @course.id, grading_period_id: period.id)
+      }.not_to raise_error
+    end
+
     it "should recompute when an assignment's points_possible changes'" do
       @group = @course.assignment_groups.create!(:name => "some group", :group_weight => 100)
       @assignment = @course.assignments.create!(:title => "Some Assignment", :points_possible => 10, :assignment_group => @group)