Add spec for removing issues.assignee_id

This migration also needs to be a post-deployment migration, as it removes a
column.

app/models/issue.rb

@@ -10,6 +10,9 @@ class Issue < ActiveRecord::Base
   include RelativePositioning
   include TimeTrackable
   include ThrottledTouch
+  include IgnorableColumn
+
+  ignore_column :assignee_id
 
   DueDateStruct = Struct.new(:title, :name).freeze
   NoDueDate     = DueDateStruct.new('No Due Date', '0').freeze

db/migrate/20170523073948_remove_assignee_id_from_issue.rb → db/post_migrate/20170523073948_remove_assignee_id_from_issue.rb

@@ -41,6 +41,8 @@ class RemoveAssigneeIdFromIssue < ActiveRecord::Migration
 
     update_value = Arel.sql('(SELECT user_id FROM issue_assignees WHERE issue_assignees.issue_id = issues.id LIMIT 1)')
 
-    update_column_in_batches(:issues, :assignee_id, update_value)
+    # This is only used in the down step, so we can ignore the RuboCop warning
+    # about large tables, as this is very unlikely to be run on GitLab.com
+    update_column_in_batches(:issues, :assignee_id, update_value) # rubocop:disable Migration/UpdateLargeTable
   end
 end

spec/migrations/remove_assignee_id_from_issue_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20170523073948_remove_assignee_id_from_issue.rb')
+
+describe RemoveAssigneeIdFromIssue, :migration do
+  let(:issues) { table(:issues) }
+  let(:issue_assignees) { table(:issue_assignees) }
+  let(:users) { table(:users) }
+
+  let!(:user_1) { users.create(email: 'email1@example.com') }
+  let!(:user_2) { users.create(email: 'email2@example.com') }
+  let!(:user_3) { users.create(email: 'email3@example.com') }
+
+  def create_issue(assignees:)
+    issues.create.tap do |issue|
+      assignees.each do |assignee|
+        issue_assignees.create(issue_id: issue.id, user_id: assignee.id)
+      end
+    end
+  end
+
+  let!(:issue_single_assignee) { create_issue(assignees: [user_1]) }
+  let!(:issue_no_assignee) { create_issue(assignees: []) }
+  let!(:issue_multiple_assignees) { create_issue(assignees: [user_2, user_3]) }
+
+  describe '#down' do
+    it 'sets the assignee_id to a random matching assignee from the assignees table' do
+      migrate!
+      disable_migrations_output { described_class.new.down }
+
+      expect(issue_single_assignee.reload.assignee_id).to eq(user_1.id)
+      expect(issue_no_assignee.reload.assignee_id).to be_nil
+      expect(issue_multiple_assignees.reload.assignee_id).to eq(user_2.id).or(user_3.id)
+
+      disable_migrations_output { described_class.new.up }
+    end
+  end
+end