How to call a class method from an instance.

rubyonrails-talk
1

I am trying to call a class method from an instance,

e.g. @item = @item.class.find(params[:id]) or @item = @item.type.find(params[:id])

@Item can reference objects form different classes.

With either of the above examples I get: undefined method `find' for NilClass:Class

Any help?

2

I don't know that it is the best way, but you can do this with eval. Eval will take a string and interpret it as ruby code.

So you would do something like:

  eval( @item.class + '.find(params[:id])' )

So this should take your class name, append the find clause and then run the result.

3

Ok I tried eval( @item.class.to_s + '.find(params[:id])' ) and i still get the error undefined method `find' for NilClass:Class error.

4

stupid mistake by me I think I know the problem, i'll post again when i'm sure.

5

I got it working with, @item = @items.pop.type.find(params[:id])

I just wasn't initializing@item with anything.

Thanks monki for the eval hint though. I wasn't aware of the eval usage.

6

Good to know, I guess I overlooked the simple answer.

Your way is better, as eval will blindly do whatever you tell it (which can be dangerous).

7

Your original method was better (IMHO) -- @item.class.find(params[:id]) -- as type is deprecated and can sometimes behave unexpectedly as it's used as the key in STI models.

Also wanted to check you knew that @items.pop is a destructive method, i.e. the item is removed from @items. I would wonder if @items.last.class.find(params[:id]) is perhaps a bit cleaner.

You've prob figured now that undefined method `find' for NilClass:Class was happening because @item was nil (instance variables are nil when they are undefined, unlike local variables -- try p @item and p item in the console), and the class of nil is NilClass, which obviously doesn't have a find method.

HTH Chris

B. Lars wrote: