allow setting a letter grade on no-points assignments

Previously this worked for 0.0 point assignments, but not assignments
where points_possible was nil. Fixes #7009

test plan:
  * create a letter grade assignment and leave the points possible blank
  * try giving a student a letter grade on both gradebook 1 and 2
  * verify that the letter grade is preserved when you refresh, and
    displays correctly to the student on their grades page

Change-Id: I8fecf57ef4fa889cb6692243b2df027eb4e79ae4
Reviewed-on: https://gerrit.instructure.com/8859
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>

app/models/assignment.rb

@@ -523,7 +523,7 @@ class Assignment < ActiveRecord::Base
       result = "#{result}%"
     elsif self.grading_type == "pass_fail"
       if self.points_possible.to_f > 0.0
-        passed = score.to_f == self.points_possible
+        passed = score.to_f == self.points_possible.to_f
       elsif given_grade
         # the score for a zero-point pass/fail assignment could be considered
         # either pass *or* fail, so look at what the current given grade is
@@ -534,8 +534,20 @@ class Assignment < ActiveRecord::Base
       end
       result = passed ? "complete" : "incomplete"
     elsif self.grading_type == "letter_grade"
-      score = score.to_f / self.points_possible
-      result = GradingStandard.score_to_grade(self.grading_scheme, score * 100)
+      if self.points_possible.to_f > 0.0
+        score = score.to_f / self.points_possible.to_f
+        result = GradingStandard.score_to_grade(self.grading_scheme, score * 100)
+      elsif given_grade
+        # the score for a zero-point letter_grade assignment could be considered
+        # to be *any* grade, so look at what the current given grade is
+        # instead of trying to calculate it
+        result = given_grade
+      else
+        # there's not really any reasonable value we can set here -- if the
+        # assignment is worth no points, and the grader didn't enter an
+        # explicit letter grade, any letter grade is as valid as any other.
+        result = GradingStandard.score_to_grade(self.grading_scheme, score.to_f)
+      end
     end
     result.to_s
   end
@@ -556,7 +568,7 @@ class Assignment < ActiveRecord::Base
     else
       # try to treat it as a letter grade
       if grading_scheme && standard_based_score = GradingStandard.grade_to_score(grading_scheme, grade)
-        (points_possible * standard_based_score).round / 100.0
+        ((points_possible || 0.0) * standard_based_score).round / 100.0
       else
         nil
       end

app/models/submission.rb

@@ -369,7 +369,7 @@ class Submission < ActiveRecord::Base
     if self.score_changed?
       @score_changed = true
       if self.assignment
-        self.grade = self.assignment.score_to_grade(self.score) if self.assignment.points_possible.to_f > 0.0 || self.assignment.grading_type != 'pass_fail'
+        self.grade = self.assignment.score_to_grade(self.score, self.grade)
       else
         self.grade = self.score.to_s
       end

spec/models/assignment_spec.rb

@@ -192,6 +192,40 @@ describe Assignment do
     @submission.user_id.should eql(@user.id)
   end
 
+  it "should preserve letter grades with zero points possible" do
+    setup_assignment_without_submission
+    @assignment.grading_type = 'letter_grade'
+    @assignment.points_possible = 0.0
+    @assignment.save!
+
+    s = @assignment.grade_student(@user, :grade => 'C')
+    s.should be_is_a(Array)
+    @assignment.reload
+    @assignment.submissions.size.should eql(1)
+    @submission = @assignment.submissions.first
+    @submission.state.should eql(:graded)
+    @submission.score.should eql(0.0)
+    @submission.grade.should eql('C')
+    @submission.user_id.should eql(@user.id)
+  end
+
+  it "should preserve letter grades with no points possible" do
+    setup_assignment_without_submission
+    @assignment.grading_type = 'letter_grade'
+    @assignment.points_possible = nil
+    @assignment.save!
+
+    s = @assignment.grade_student(@user, :grade => 'C')
+    s.should be_is_a(Array)
+    @assignment.reload
+    @assignment.submissions.size.should eql(1)
+    @submission = @assignment.submissions.first
+    @submission.state.should eql(:graded)
+    @submission.score.should eql(0.0)
+    @submission.grade.should eql('C')
+    @submission.user_id.should eql(@user.id)
+  end
+
   it "should give a grade to extra credit assignments" do
     setup_assignment_without_submission
     @assignment.grading_type = 'points'

spec/selenium/gradebook2_spec.rb

@@ -341,6 +341,23 @@ describe "gradebook2" do
     driver.execute_script '$.store.clear();'
   end
 
+  it "should allow setting a letter grade on a no-points assignment" do
+    assignment_model(:course => @course, :grading_type => 'letter_grade', :points_possible => nil, :title => 'no-points')
+    get "/courses/#{@course.id}/gradebook2"
+    wait_for_ajaximations
+
+    edit_grade(driver.find_element(:css, '#gradebook_grid [row="0"] .l3'), 'A-')
+
+    get "/courses/#{@course.id}/gradebook2"
+    wait_for_ajaximations
+
+    driver.find_element(:css, '#gradebook_grid [row="0"] .l3').text.should == 'A-'
+    @assignment.submissions.size.should == 1
+    sub = @assignment.submissions.first
+    sub.grade.should == 'A-'
+    sub.score.should == 0.0
+  end
+
   it "should change grades and validate course total is correct" do
     expected_edited_total = "33.3%"