Check if there are pending migrations without loading the Rails app, for speedy checks. Store in the script directory.

pending_migration_check.rb

#!/usr/bin/env ruby

# Consider loading the db driver directly instead of relying on relatively slow bundler
require 'bundler'
Bundler.setup

require 'rails'
require 'active_record'

# Load the Rails configuration without loading the application
config = Rails::Application::Configuration.new
config.root = File.expand_path(File.join(__FILE__, '../..'))

# Create a model to access already-run migration versions
class SchemaMigration < ActiveRecord::Base; end
SchemaMigration.configurations = config.database_configuration
SchemaMigration.establish_connection

# Calculate which if any migrations are pending
migrated = Set.new(SchemaMigration.all.map(&:version))
migrations = Set.new(Dir.glob('db/migrate/[0-9]*').map { |path| path.sub(/\D*(\d+)\D*/, '\1')})
pending = migrations-migrated

# Output the result
if pending.any?
  puts "Pending migrations: #{pending.to_a.join(', ')}"
  exit 1
else
  puts "No pending migrations"
  exit 0
end