How to transform a string into a variable/field?

@zoo.attributes gives you a hash of the object attributes. So you can access them like

@zoo.attributes['monkey']

This will give nil if the attribute is not present. Calling a method which doesn't exist will throw NoMethodError

In your controller you could use the public_send (or even send) method like this:

def your_action
  @zoo.public_send(params[:your_field])
end

Obviously this is no good, since someone can post somehing like delete_all as the method name, so you must sanitize the value you get from the form. As a simple example:

ALLOWED_METHODS = [:monkey, :tiger]

def your_action
  raise unless ALLOWED_METHODS.include?(params[:your_field])
  @zoo.public_send(params[:your_field])
end

There is much better way to do this - you should use Object#send or (even better, because it raises error if you try to call private or protected method) Object#public_send, like this:

message = 'monkey'
@zoo.public_send( message )

You could implement method_missing in your class and have it interrogate @zoo for a matching method. Documentation: http://ruby-doc.org/core-1.9.3/BasicObject.html#method-i-method_missing

require 'ostruct' # only necessary for my example

class ZooKeeper

  def initialize
    @zoo = OpenStruct.new(monkey: 'chimp')
  end

  def method_missing(method, *args)
    if @zoo.respond_to?(method)
      return @zoo.send(method)
    else
      super
    end
  end
end

keeper = ZooKeeper.new
keeper.monkey  #=> "chimp"
keeper.lion    #=> NoMethodError: undefined method `lion'