master courses: fix sync when old associations are deleted

Change-Id: Ice68bbb549ef745730bd2bc05146567c4b412b9a
Reviewed-on: https://gerrit.instructure.com/112440
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/models/master_courses/master_migration.rb

@@ -118,6 +118,7 @@ class MasterCourses::MasterMigration < ActiveRecord::Base
   end
 
   def export_to_child_courses(type, subscriptions, export_is_primary)
+    @export_type = type
     if type == :selective
       @deletions = self.master_template.deletions_since_last_export
       @creations = {} # will be populated during export
@@ -177,7 +178,7 @@ class MasterCourses::MasterMigration < ActiveRecord::Base
   end
 
   def add_exported_asset(asset)
-    return unless last_export_at
+    return unless @export_type == :selective
     @export_count += 1
     return if @export_count > Setting.get('master_courses_history_count', '150').to_i
     set = asset.created_at >= last_export_at ? @creations : @updates

spec/models/master_courses/master_migration_spec.rb

@@ -726,6 +726,21 @@ describe MasterCourses::MasterMigration do
       [topic_override, normal_override].each { |ao| expect(ao.reload).to be_deleted }
     end
 
+    it "should work with a single full export for a new association" do
+      @copy_to1 = course_factory
+      sub1 = @template.add_child_course!(@copy_to1)
+      topic = @copy_from.discussion_topics.create!(:title => "some title")
+
+      run_master_migration
+
+      sub1.destroy!
+      @copy_to2 = course_factory
+      @template.add_child_course!(@copy_to2)
+
+      run_master_migration
+      expect(@copy_to2.discussion_topics.first).to be_present
+    end
+
     context "master courses + external migrations" do
       class TestExternalContentService
         cattr_reader :course, :imported_content