如何在类上使用哈希键作为方法？

Question

I have a class and a hash. How can I get the members of the hash to dynamically become methods on the class with the key as the method name?

class User
  def initialize
    @attributes = {"sn" => "Doe", "givenName" => "John"}
  end
end

For example, I would like to be able to have the following output Doe :

u = User.new
puts u.sn

Answer 1

Just use OpenStruct:

require 'ostruct'
class User < OpenStruct
end

u = User.new :sn => 222
u.sn

Answer 2

def method_missing(name, *args, &blk)
  if args.empty? && blk.nil? && @attributes.has_key?(name)
    @attributes[name]
  else
    super
  end
end

Explanation: If you call a method, which does not exist, method_missing is invoked with the name of the method as the first parameter, followed by the arguments given to the method and the block if one was given.

In the above we say that if a method, which was not defined, is called without arguments and without a block and the hash has an entry with the method name as key, it will return the value of that entry. Otherwise it will just proceed as usual.

Answer 3

The solution by sepp2k is the way to go. However, if your @attributes never change after the initialization and you need speed, then you could do it in this way:

class User
  def initialize
    @attributes = {"sn" => "Doe", "givenName" => "John"}
    @attributes.each do |k,v|
      self.class.send :define_method, k do v end
    end
  end
end

User.new.givenName # => "John"

This generates all the methods in advance...

Answer 4

Actually severin have a better idea, just because usage of method_missing is a bad practice, not all the time, but most of it.

One problem with that code provided by severin : it returns value that have been passed to initializer, so you cannot change it. I suggest you a little different approach:

class User < Hash
  def initialize(attrs)
    attrs.each do |k, v|
      self[k] = v
    end
  end

  def []=(k, v)
    unless respond_to?(k)
      self.class.send :define_method, k do
        self[k]
      end
    end

    super
  end
end

Lets check it:

u = User.new(:name => 'John')
p u.name
u[:name] = 'Maria'
p u.name

And also you can do it with Struct:

attrs = {:name => 'John', :age => 22, :position => 'developer'}
keys = attrs.keys

user = Struct.new(*keys).new(*keys.map { |k| attrs[k] })

Lets test it:

p user
p user.name
user[:name] = 'Maria'
p user.name
user.name = 'Vlad'
p user[:name]

Or even OpenStruct, but be careful it will not create method if it already have it in instance methods, you can look for that by using OpenStruct.instance_methods (because of type is used, I'm now using second approach):

attrs = {:name => 'John', :age => 22, :position => 'developer'}
user = OpenStruct.new(attrs)

Yep, so easy:

user.name
user[:name] # will give you an error, because OpenStruct isn't a Enumerable or Hash

Answer 5

You can "borrow" ActiveResource for this. It even handles nested hashes and assignment:

require 'active_resource'
class User < ActiveResource::Base
  self.site = ''  # must be a string
end

Usage:

u = User.new "sn" => "Doe", "givenName" => "John", 'job'=>{'description'=>'Engineer'}
u.sn  # => "Doe"
u.sn = 'Deere'
u.job.description  # => "Engineer"
# deletion
u.attributes.delete('givenName')

Note that u.job is a User::Job - this class is auto-created. There's a gotcha when assigning to a nested value. You can't just assign a hash, but must wrap it in the appropriate class:

u.job = User::Job.new 'foo' => 'bar'
u.job.foo  # => 'bar

Unfortunately, when you want to add a nested hash that doesn't have a corresponding class, it's uglier because you have to force ARes to create the class from the hash:

# assign the hash first
u.car = {'make' => 'Ford'}
# force refresh - this can be put into a method
u = User.new Hash.from_xml(u.to_xml).values.first