成功安全的 constantize

User 是 ActiveRecord 或 Mongoid 类。将 User 替换为项目中的任何 Rails 类（甚至类似 Integer 或 Array）

my_string = "User" # Capitalized string
  # => 'User'
my_constant = my_string.safe_constantize
  # => User
my_constant.all.count
  # => 18

my_string = "Array"
  # => 'Array'
my_constant = my_string.safe_constantize
  # => Array
my_constant.new(4)
  # => [nil, nil, nil, nil]