Question

I have three objects that are all the same class. One was created via Item.new and the other two were pulled from the database (Mongoid). I’m passing one/any of these objects to another method and checking the type in that method via is_a?:

def initialize (item, attrs = nil, options = nil)
  super(attrs, options)
  raise 'invalid item object' unless item.is_a?(Item)

Well, this raise is getting hit. So I check the class, is_a and instance_of in rails console. I’m getting conflicting results. Why would they have the same class but only one of them be an instance_of that class?

>> i0.is_a? Item
=> false
>> i1.is_a? Item
=> false
>> i2.is_a? Item
=> true

>> i0.class
=> Item
>> i1.class
=> Item
>> i2.class
=> Item

>> i0.instance_of?(Item)
=> false
>> i1.instance_of?(Item)
=> false
>> i2.instance_of?(Item)
=> true

Is there a better way to do this type checking of my inputs? Why would three things that are the same class not all be instances of that class?

Answer 1

I don’t know Mongoid, but usually, in a DB access library, you don’t get the actual object out of the database but rather a proxy object that acts as a stand-in for the object stored in the DB. Since Ruby lacks the features to implement a perfect transparent proxy, you will sometimes see odd results, especially when using reflection or around object identity.