add tests

test/models/saved_annotation_test.rb

@@ -18,4 +18,28 @@ class SavedAnnotationTest < ActiveSupport::TestCase
   # test "the truth" do
   #   assert true
   # end
+
+  test 'filtering by course_id should contain nil values' do
+    user = create :user
+    course = create :course
+    s1 = create :saved_annotation, course: nil, user: user
+    s2 = create :saved_annotation, course: course, user: user
+    s3 = create :saved_annotation, course: create(:course), user: user
+
+    assert_includes SavedAnnotation.by_course(course), s1
+    assert_includes SavedAnnotation.by_course(course), s2
+    assert_not_includes SavedAnnotation.by_course(course), s3
+  end
+
+  test 'filtering by exercise_id should contain nil values' do
+    user = create :user
+    exercise = create :exercise
+    s1 = create :saved_annotation, exercise: nil, user: user
+    s2 = create :saved_annotation, exercise: exercise, user: user
+    s3 = create :saved_annotation, exercise: create(:exercise), user: user
+
+    assert_includes SavedAnnotation.by_exercise(exercise), s1
+    assert_includes SavedAnnotation.by_exercise(exercise), s2
+    assert_not_includes SavedAnnotation.by_exercise(exercise), s3
+  end
 end