don't overwrite assignment group settings on initial bp sync

test plan:
 - create assignment groups with group weights / drop rules in
   Course B, a blueprint course
 - add some assignments to the groups
 - create assignment groups with the same names but different
   properties in Course C, a new course
 - put some different assignments there
 - associate Course C with Course B and wait for the initial
   sync to run
 - Course C should keep its original assignment group weights
   and drop rules, and the assignments in both groups
   should be merged
 - repeat with a new course but _don't_ set any assignment group
   rules in the target course. it should inherit the blueprint
   course's assignment group rules

flag = none

fixes LS-1109

Change-Id: I0e290e2a85d83031d3c44437df5a6fe5d46c6b7d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/242828
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: James Williams <jamesw@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/models/importers/assignment_group_importer.rb

@@ -75,7 +75,7 @@ module Importers
       return nil if hash[:migration_id] && hash[:assignment_groups_to_import] && !hash[:assignment_groups_to_import][hash[:migration_id]]
       item ||= AssignmentGroup.where(context_id: context, context_type: context.class.to_s, id: hash[:id]).first
       item ||= AssignmentGroup.where(context_id: context, context_type: context.class.to_s, migration_id: hash[:migration_id]).first if hash[:migration_id]
-      item ||= context.assignment_groups.where(name: hash[:title], migration_id: nil).first
+      item ||= match_assignment_group_by_name(context, migration, hash[:title])
       item ||= context.assignment_groups.temp_record
       migration.add_imported_item(item)
       item.saved_by = :migration
@@ -106,5 +106,21 @@ module Importers
       item.save!
       item
     end
+
+    def self.match_assignment_group_by_name(context, migration, name)
+      ag = context.assignment_groups.where(name: name, migration_id: nil).first
+      if ag && migration.for_master_course_import?
+        # prevent overwriting assignment group settings in a pre-existing group that was matched by name
+        downstream_changes = []
+        downstream_changes << 'group_weight' if ag.group_weight&.> 0
+        downstream_changes << 'rules' if ag.rules.present?
+        if downstream_changes.any?
+          tag = migration.master_course_subscription&.content_tag_for(ag)
+          tag.downstream_changes |= downstream_changes
+          tag.save!
+        end
+      end
+      ag
+    end
   end
 end

spec/models/master_courses/master_migration_spec.rb

@@ -584,6 +584,27 @@ describe MasterCourses::MasterMigration do
       expect(ag1_to.rules).to eq "drop_lowest:3\n"
     end
 
+    it "doesn't overwrite weights and rules of an assignment group with a similar name on initial sync" do
+      @copy_to = course_factory
+      sub = @template.add_child_course!(@copy_to)
+
+      ag1 = @copy_from.assignment_groups.create!(:name => "group1", :group_weight => 50)
+      a1 = @copy_from.assignments.create!(:title => "assmt1", :assignment_group => ag1)
+      ag1.update_attribute(:rules, "drop_lowest:1\nnever_drop:#{a1.id}\n")
+
+      ag1_assimilation_target = @copy_to.assignment_groups.create!(:name => "group1", :group_weight => 33)
+      ag1_assimilation_target.update_attribute(:rules, "drop_highest:1\n")
+
+      run_master_migration
+
+      ag1_to = @copy_to.assignment_groups.where(:migration_id => mig_id(ag1)).first
+      expect(ag1_to).to eq ag1_assimilation_target
+      expect(ag1_to.group_weight).to eq 33
+      expect(ag1_to.rules).to eq "drop_highest:1\n"
+      a1_to = ag1_to.assignments.first
+      expect(a1_to).to be
+    end
+
     it "should sync unpublished quiz points possible" do
       @copy_to = course_factory
       sub = @template.add_child_course!(@copy_to)