See the difference

Yesterday I faced the strange behaviour of the array wrapping in context of Arel. I tried to wrap something like this User.arel_table[:id] but Array(User.arel_table[:id]) and Array.wrap(User.arel_table[:id]) gave me different results. Now I’ll tell you why you have to know about some differences in array wrapping and why Rails contains their own wrap realization.

You probably know about Array.wrap in Rails but if you don’t here you are:

def self.wrap(object)
  if object.nil?
    []
  elsif object.respond_to?(:to_ary)
    object.to_ary || [object]
  else
    [object]
  end
end

And you probably know about Kernel#Array:

VALUE
rb_Array(VALUE val) {
  VALUE tmp = rb_check_array_type(val);

  if (NIL_P(tmp)) {
    tmp = rb_check_convert_type(val, T_ARRAY, "Array", "to_a");
    if (NIL_P(tmp)) {
      return rb_ary_new3(1, val);
    }
  }
  return tmp;
}

static VALUE
rb_f_array(VALUE obj, VALUE arg) {
  return rb_Array(arg);
}

Let’s talk about ruby realization first of all. When you invoke Array(object), ruby will try to convert object into array by means of rb_check_array_type(val) call. At the first step this function will try to invoke to_ary and if it’s defined and result differ from nil and the same class as and Array it’ll be returned. The second step (Line 6) (if result of the first step was nil) is to_a, here is the same thing as described above. The third and final step (Line 8) (if steps above return nil) is new array will be created with object as its element.

Now take a look at Rails realization. If object is nil it returns empty array. If object responds to to_ary method it returns the result or if the result is nil just [object]. And finally it returns [object].

You see that Rails method doesn’t call to_a. And one thing I haven’t mentioned is raise. Yep. Ruby version will raise exception if object that you return in to_ary or to_a methods isn’t Array.

Now you and me know why Array(object) returns me not what I wanted because of overridden to_a method in Arel. And in my case I select Rails version of course.

There are things I have never thought about and just used them. They seemed to me very simple but in reality they are tiniest bits of a big complicated mechanism. We use too many abstraction levels and work on the top of it and rely on it. You should be waiting for troubles from everywhere.