Ruby Metaprogramming: dynamic instance variable names

Let's say I have the following hash:

{ :foo => 'bar', :baz => 'qux' }

How could I dynamically set the keys and values to become instance variables in an object...

class Example
  def initialize( hash )
    ... magic happens here...
  end
end

... so that I end up with the following inside the model...

@foo = 'bar'
@baz = 'qux'

?

The method you are looking for is instance_variable_set. So:

hash.each { |name, value| instance_variable_set(name, value) }

Or, more briefly,

hash.each &method(:instance_variable_set)

If your instance variable names are missing the "@" (as they are in the OP's example), you'll need to add them, so it would be more like:

hash.each { |name, value| instance_variable_set("@#{name}", value) }

h = { :foo => 'bar', :baz => 'qux' }

o = Struct.new(*h.keys).new(*h.values)

o.baz
 => "qux" 
o.foo
 => "bar" 

You make we want to cry :)

In any case, see Object#instance_variable_get and Object#instance_variable_set.

Happy coding.

You can also use send which prevents the user from setting non-existent instance variables:

def initialize(hash)
  hash.each { |key, value| send("#{key}=", value) }
end

Use send when in your class there is a setter like attr_accessor for your instance variables:

class Example
  attr_accessor :foo, :baz
  def initialize(hash)
    hash.each { |key, value| send("#{key}=", value) }
  end
end