Find discrepancies between Rails schema.rb and the actual database

fake_schema.rb

class FakeSchema
  attr_reader :defs, :diff

  def self.define(*, &block)
    $schema = new
    $schema.instance_eval(&block)
  end

  def initialize
    @defs = {}
  end

  def create_table(name, options, &block)
    @defs[name] = {}
    yield table = FakeTable.new(options)
    @defs[name][:columns] = table.columns
    nil
  end

  def add_index(table_name, fields, options)
    @defs[table_name][:indexes] ||= []
    @defs[table_name][:indexes] << options.fetch(:name)
    nil
  end

  def compare
    @diff = Hash.new { |me, key| me[key] = HashArray.new }
    @defs.each do |table, options|
      db_cols = db.select_values("SHOW FIELDS FROM #{table}")
      if db_cols.sort != options[:columns].sort
        @diff[table][:columns] = db_cols - options[:columns]
        @diff[table][:columns] |= options[:columns] - db_cols
      end
      Array(options[:indexes]).each do |index|
        if db.select_values("SHOW KEYS FROM #{table} WHERE key_name = '#{index}'").blank?
          @diff[table][:indexes] << index
        end
      end
    end
    @diff.empty?
  end

  def db
    @db ||= ActiveRecord::Base.connection
  end

  class FakeTable
    attr_reader :columns

    def initialize(options)
      @columns = []
      @columns << 'id' if options[:id] != false
    end

    def method_missing(name, *args, &block)
      @columns << args.first
    end
  end
end
ActiveRecord::Schema = FakeSchema
require Rails.root.join('db/schema')
$schema.compare