add override_grade field to GradesType

closes GRADE-2003

Test Plan
- Enable Final Grade Overrides feature.
- Enable a grading scheme.
- Assign an override grade to one student, and have another student
  on hand that does not have an override grade.
- Run a query for override scores/grades on both students:

query {
  legacyNode(_id: `enrollment_id`, type: Enrollment) {
    ... on Enrollment {
      user {
        name
      }
      grades {
        overrideScore
        overrideGrade
      }
    }
  }
}

- Verify that the student with an override grade has it here.
- Verify that the student without an override grade does not have it
  here.

- Disable the grading scheme and verify that the override grade is
  null.

Change-Id: I8b7e824095c044e24f08e59f21d25347a3f3b9b9
Reviewed-on: https://gerrit.instructure.com/181747
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Reviewed-by: Keith Garner <kgarner@instructure.com>
Tested-by: Jenkins
QA-Review: Gary Mei <gmei@instructure.com>
Product-Review: Keith Garner <kgarner@instructure.com>

app/graphql/types/grades_type.rb

@@ -46,6 +46,14 @@ module Types
       The override score. Supersedes the computed final score if set.
     DESC
 
+    field :override_grade, String, <<~DESC, null: true
+      The override grade. Supersedes the computed final grade if set.
+    DESC
+    def override_grade
+      return nil if override_score.blank?
+      object.effective_final_grade
+    end
+
     field :grading_period, GradingPeriodType, null: true
     def grading_period
       load_association :grading_period

schema.graphql

@@ -836,6 +836,11 @@ type Grades {
   finalScore: Float
   gradingPeriod: GradingPeriod
 
+  """
+  The override grade. Supersedes the computed final grade if set.
+  """
+  overrideGrade: String
+
   """
   The override score. Supersedes the computed final score if set.
   """

spec/graphql/types/grades_type_spec.rb

@@ -23,6 +23,7 @@ describe Types::GradesType do
   let!(:account) { Account.create! }
   let!(:course) { account.courses.create!(grading_standard_enabled: true) }
   let!(:student_enrollment) { course.enroll_student(User.create!, enrollment_state: 'active') }
+  let!(:score) { student_enrollment.find_score(grading_period_id: grading_period.id) }
   let!(:grading_period) do
     group = account.grading_period_groups.create!(title: "a test group")
     group.enrollment_terms << course.enrollment_term
@@ -39,7 +40,6 @@ describe Types::GradesType do
   let(:enrollment_type) { GraphQLTypeTester.new(student_enrollment, current_user: teacher) }
 
   before(:each) do
-    score = student_enrollment.find_score(grading_period_id: grading_period.id)
     score.update!(
       current_score: 68.0,
       final_score: 78.1,
@@ -66,6 +66,20 @@ describe Types::GradesType do
       expect(resolve_grades_field("overrideScore")).to eq 88.2
     end
 
+    it "resolves the overrideGrade field to the corresponding Score's override grade" do
+      expect(resolve_grades_field("overrideGrade")).to eq "B+"
+    end
+
+    it "resolves the overrideGrade field to nil when grading standards are not enabled" do
+      course.update!(grading_standard_id: nil)
+      expect(resolve_grades_field("overrideGrade")).to be nil
+    end
+
+    it "resolves the overrideGrade field to nil when an override score is not present" do
+      score.update!(override_score: nil)
+      expect(resolve_grades_field("overrideGrade")).to be nil
+    end
+
     it "resolves the unpostedCurrentScore field to the corresponding Score's unposted_current_score" do
       expect(resolve_grades_field("unpostedCurrentScore")).to eq 71.3
     end