Add spec for multiple assignments

ee/spec/services/ee/merge_requests/update_assignees_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe MergeRequests::UpdateAssigneesService do
+  let_it_be(:group) { create(:group, :public) }
+  let_it_be(:project) { create(:project, :private, :repository, group: group) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:user2) { create(:user) }
+  let_it_be(:user3) { create(:user) }
+
+  let_it_be_with_reload(:merge_request) do
+    create(:merge_request, :simple, :unique_branches,
+           assignee_ids: [user.id],
+           source_project: project,
+           author: user)
+  end
+
+  before do
+    project.add_maintainer(user)
+    project.add_developer(user2)
+    project.add_developer(user3)
+  end
+
+  let(:service) { described_class.new(project: project, current_user: user, params: opts) }
+
+  describe 'execute' do
+    def update_merge_request
+      service.execute(merge_request)
+      merge_request.reload
+    end
+
+    context 'when the parameters are valid' do
+      context 'when using sentinel values' do
+        let(:opts) { { assignee_ids: [0, 0, 0] } }
+
+        it 'removes all assignees' do
+          expect { update_merge_request }.to change(merge_request, :assignees).to([])
+        end
+      end
+
+      context 'the assignee_ids parameter is the empty list' do
+        let(:opts) { { assignee_ids: [] } }
+
+        it 'removes all assignees' do
+          expect { update_merge_request }.to change(merge_request, :assignees).to([])
+        end
+      end
+
+      context 'the assignee_ids parameter contains both zeros and valid IDs' do
+        let(:opts) { { assignee_ids: [0, user2.id, 0, user3.id, 0] } }
+
+        it 'ignores 0 IDs' do
+          expect { update_merge_request }.to change(merge_request, :assignees).to([user2, user3])
+        end
+      end
+    end
+  end
+end