enforce no changing group_category on assignments after submissions

refs CNVS-11813

Test plan:
  * create an assignment
  * assign a group category to it (this should work)
  * remove the group category (this should work)
  * submit some homework
  * use the API to try and change the group category.  it won't let you

Change-Id: I09a56e281956d30e9213d6105bd9c75ede97da1c
Reviewed-on: https://gerrit.instructure.com/32719
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>

app/models/assignment.rb

@@ -61,6 +61,7 @@ class Assignment < ActiveRecord::Base
   has_one :external_tool_tag, :class_name => 'ContentTag', :as => :context, :dependent => :destroy
   validates_associated :external_tool_tag, :if => :external_tool?
   validate :validate_draft_state_change, :if => :workflow_state_changed?
+  validate :group_category_changes_ok?
 
   accepts_nested_attributes_for :external_tool_tag, :update_only => true, :reject_if => proc { |attrs|
     # only accept the url and new_tab params, the other accessible
@@ -82,6 +83,15 @@ class Assignment < ActiveRecord::Base
     true
   end
 
+  def group_category_changes_ok?
+    return if new_record?
+    return unless has_submitted_submissions?
+    if group_category_id_changed?
+      errors.add :group_category_id, I18n.t("group_category_locked",
+                                            "The group category can't be changed because students have already submitted on this assignment")
+    end
+  end
+
   API_NEEDED_FIELDS = %w(
     id
     title

spec/models/assignment_spec.rb

@@ -2638,6 +2638,44 @@ describe Assignment do
       @assignment.save!
     end
   end
+
+  describe "group category validation" do
+    before do
+      student_in_course active_all: true
+      @group_category = @course.group_categories.create! name: "groups"
+      @groups = 2.times.map { |i|
+        @group_category.groups.create! name: "group #{i}", context: @course
+      }
+    end
+
+    def assignment(group_category = nil)
+      a = @course.assignments.build name: "test"
+      a.group_category = group_category
+      a.tap &:save!
+    end
+
+    it "lets you change group category attributes before homework is submitted" do
+      a1 = assignment
+      a1.group_category = @group_category
+      a1.should be_valid
+
+      a2 = assignment(@group_category)
+      a2.group_category = nil
+      a2.should be_valid
+    end
+
+    it "doesn't let you change group category attributes after homework is submitted" do
+      a1 = assignment
+      a1.submit_homework @student, body: "hello, world"
+      a1.group_category = @group_category
+      a1.should_not be_valid
+
+      a2 = assignment(@group_category)
+      a2.submit_homework @student, body: "hello, world"
+      a2.group_category = nil
+      a2.should_not be_valid
+    end
+  end
 end
 
 def setup_assignment_with_group