Refactor a method so the Assignments#show action is faster

In AssignmentsController#show we [email protected]_assignment_group
which was causing this to be called and also causing a:
SELECT "assignment_groups".* 
FROM "assignment_groups" 
WHERE "assignment_groups"."context_id" = 1 
	AND "assignment_groups"."context_type" = 'Course' 
	AND (assignment_groups.workflow_state<>'deleted') 
ORDER BY assignment_groups.position, 
    CAST(LOWER(replace(assignment_groups.name, '\', '\\')) AS bytea)

…even if the assignment already had an assignment_group_id

We should be able to avoid doing that query if we already have an
assignment_group_id

Test plan:
* for an assignment that already has an assignment_grou_id
* go to AssignmentsController#show (with or without a2 on)
* it should not run that ^ query

Change-Id: I64d1d845c50a4cf3e56d2b750954c5a8a778c265
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218101
Tested-by: Service Cloud Jenkins <[email protected]>
Tested-by: Jenkins
Reviewed-by: Steven Burnett <[email protected]>
QA-Review: Ryan Shaw <[email protected]>
Product-Review: Ryan Shaw <[email protected]>

app/models/assignment.rb

@@ -894,13 +894,11 @@ def default_values
   protected :default_values
 
   def ensure_assignment_group(do_save = true)
+    return if assignment_group_id
     self.context.require_assignment_group
-    assignment_groups = self.context.assignment_groups.active
-    if !assignment_groups.map(&:id).include?(self.assignment_group_id)
-      self.assignment_group = assignment_groups.first
-      Shackles.activate(:master) do
-        save! if do_save
-      end
+    self.assignment_group = self.context.assignment_groups.active.first
+    if do_save
+      Shackles.activate(:master) { save! }
     end
   end