assignments with no preview should show 'No Preview Available'

refs #7425

Marc L. pointed out that this wasn't working.

test plan:
- create a new assignment with paper submission
- give the assignment a score for a student in the speedgrader
- as the student verify that the submission details page shows
  'No Preview Available'

Change-Id: Ie29943589a1a50ca7a14a79d1e1d5e0556ece1f3
Reviewed-on: https://gerrit.instructure.com/10846
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>

app/models/assignment.rb

@@ -1642,6 +1642,10 @@ class Assignment < ActiveRecord::Base
     submission_types && submission_types.strip != "" && submission_types != "none" && submission_types != 'not_graded' && submission_types != "on_paper" && submission_types != 'external_tool'
   end
 
+  def expects_external_submission?
+    submission_types == 'on_paper' || submission_types == 'external_tool'
+  end
+
   def <=>(comparable)
     sort_key <=> comparable.sort_key
   end

app/views/submissions/show_preview.html.erb

@@ -179,7 +179,7 @@
     });
     </script>
   <% end %>
-<% elsif @submission.has_submission? %>
+<% elsif @submission.has_submission? || @assignment.expects_external_submission? %>
   <div style="margin: 20px auto; width: 400px; height: 50px; font-size: 1.5em;  font-weight: bold;">
     <%= t('no_preview_available', 'No Preview Available') %>
   </div>

spec/views/submissions/show_preview.html.erb_spec.rb

@@ -29,5 +29,15 @@ describe "/submissions/show_preview" do
     render "submissions/show_preview"
     response.should_not be_nil
   end
+
+  it "should give a user-friendly explaination why there's no preview" do
+    course_with_student
+    view_context
+    a = @course.assignments.create!(:title => "some assignment", :submission_types => 'on_paper')
+    assigns[:assignment] = a
+    assigns[:submission] = a.submit_homework(@user)
+    render "submissions/show_preview"
+    response.body.should match /No Preview Available/
+  end
 end