How to instantiate class from name string in Rails?

问题

How we can instantiate class from it's name string in Ruby-on-Rails?

For example we have it's name in database in format like "ClassName" or "my_super_class_name".

How we can create object from it?

Solution:

Was looking for it myself, but not found, so here it is. Ruby-on-Rails API Method

name = "ClassName"
instance = name.constantize.new

It can be even not formatted, we can user string method .classify

name = "my_super_class"
instance = name.classify.constantize.new

Of course maybe this is not very 'Rails way', but it solves it's purpose.

回答1:

klass = Object.const_get "ClassName"

about class methods

class KlassExample
    def self.klass_method
        puts "Hello World from Class method"
    end
end
klass = Object.const_get "KlassExample"
klass.klass_method

irb(main):061:0> klass.klass_method
Hello World from Class method

回答2:

Others may also be looking for an alternative that does not throw an error if it fails to find the class. safe_constantize is just that.

class MyClass
end

"my_class".classify.safe_constantize.new #  #<MyClass:0x007fec3a96b8a0>
"omg_evil".classify.safe_constantize.new #  nil

回答3:

You can simply convert a string and initialize a class out of it by:

klass_name = "Module::ClassName"
klass_name.constantize

To initialize a new object:

klass_name.constantize.new

I hope this turns out to be helpful. Thanks!

回答4:

I'm surprised nobody is considering security and hacking in their responses. Instantiation of an arbitrary string that likely came even indirectly from user input is asking for trouble and hacking. We all should/must be whitelisting unless we're sure the string is fully controlled and monitored

def class_for(name)
  {
    "foo" => Foo,
    "bar" => Bar,
  }[name] || raise UnknownClass
end

class_for(name_wherever_this_came_from).create!(params_somehow)

How you would know the appropriate params arbitrarily without having a whitelist would be challenging but you get the idea.

来源：https://stackoverflow.com/questions/14070369/how-to-instantiate-class-from-name-string-in-rails

标签

ruby-on-rails

ruby-on-rails-3

string

class

instantiation