Fix Groups Import issue that caused old groups memberships to persist

closes VICE-2382

flag=none

test plan:
  - Specs pass
  - Go to ticket and follow reproduce steps
  - After each import, groups membership should be as
      stated on the CSV.

qa risk: low

Change-Id: Ibaa98e165f4f5284caae0bc02011c2c76106ebd7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/282770
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Drake Harper <drake.harper@instructure.com>
Product-Review: Drake Harper <drake.harper@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>

app/models/group_and_membership_importer.rb

@@ -109,7 +109,7 @@ class GroupAndMembershipImporter < ActiveRecord::Base
 
   def validate_user(user)
     # if they have any memberships, we are moving them via delete and add
-    GroupMembership.where(group_id: group_category.groups.select(:id), user_id: user.id).take&.destroy
+    GroupMembership.where(group_id: group_category.groups.select(:id), user_id: user.id).destroy_all
   end
 
   def user_from_row(row)

spec/models/group_and_membership_importer_spec.rb

@@ -90,6 +90,52 @@ describe GroupAndMembershipImporter do
       expect(progress.workflow_state).to eq "completed"
     end
 
+    it "works multiple times" do
+      import_csv_data(%(user_id,group_name
+                        user_0, first group
+                        user_1, second group
+                        user_2, third group
+                        user_3, third group
+                        user_4, first group))
+
+      expect(Pseudonym.where(user: gc1.groups.where(name: "first group").take.users).pluck(:sis_user_id).sort).to eq %w[user_0 user_4]
+      expect(Pseudonym.where(user: gc1.groups.where(name: "second group").take.users).pluck(:sis_user_id).sort).to eq ["user_1"]
+      expect(Pseudonym.where(user: gc1.groups.where(name: "third group").take.users).pluck(:sis_user_id).sort).to eq %w[user_2 user_3]
+
+      import_csv_data(%(user_id,group_name
+                        user_0, first group
+                        user_1, first group
+                        user_2, first group
+                        user_3, third group
+                        user_4, third group))
+
+      expect(Pseudonym.where(user: gc1.groups.where(name: "first group").take.users).pluck(:sis_user_id).sort).to eq %w[user_0 user_1 user_2]
+      expect(Pseudonym.where(user: gc1.groups.where(name: "second group").take.users).pluck(:sis_user_id).sort).to eq []
+      expect(Pseudonym.where(user: gc1.groups.where(name: "third group").take.users).pluck(:sis_user_id).sort).to eq %w[user_3 user_4]
+
+      import_csv_data(%(user_id,group_name
+                        user_0, third group
+                        user_1, third group
+                        user_2, third group
+                        user_3, third group
+                        user_4, third group))
+
+      expect(Pseudonym.where(user: gc1.groups.where(name: "first group").take.users).pluck(:sis_user_id).sort).to eq []
+      expect(Pseudonym.where(user: gc1.groups.where(name: "second group").take.users).pluck(:sis_user_id).sort).to eq []
+      expect(Pseudonym.where(user: gc1.groups.where(name: "third group").take.users).pluck(:sis_user_id).sort).to eq %w[user_0 user_1 user_2 user_3 user_4]
+
+      import_csv_data(%(user_id,group_name
+                        user_0, first group
+                        user_1, first group
+                        user_2, first group
+                        user_3, first group
+                        user_4, first group))
+
+      expect(Pseudonym.where(user: gc1.groups.where(name: "first group").take.users).pluck(:sis_user_id).sort).to eq %w[user_0 user_1 user_2 user_3 user_4]
+      expect(Pseudonym.where(user: gc1.groups.where(name: "second group").take.users).pluck(:sis_user_id).sort).to eq []
+      expect(Pseudonym.where(user: gc1.groups.where(name: "third group").take.users).pluck(:sis_user_id).sort).to eq []
+    end
+
     it "skips invalid_users" do
       progress = import_csv_data(%(user_id,group_name
                                    user_0, first group