ActiveRecord and a custom method_missing

06 September 2007 by Mathias Meyer

Don''t be tempted to overwrite method_missing in an ActiveRecord-based model class. It will open a can of worms that's hard to close without removing your custom version again.

A lot of stuff in ActiveRecord is based on it, for example all the beautiful finder methods like find_all_by_this_field_and_that_field or simply setting and getting of attribute values. While associations get their own methods, the plain attributes are routed through method_missing. So @user.name and @user.name = "Average Joe" all go through it. You can try that out by overwriting method_missing, strictly for educational purposes of course, and only if you promise to remove it afterwards.

alias :original_method_missing :method_missing
def method_missing(method_name, *args)
  puts method_name
  original_method_missing method_name, args
end

You'd think that this code shouldn't break anything. I tried it, and the validations stopped working. Since there's a lo-hot going on in ActiveRecord, I haven't dug in yet to have a look why that's the case, again strictly for educational purposes. But I'm curious for sure.

If you want to bring dynamic code into your classes, for example generated methods, you're better off generating the code at runtime, just like ActiveRecord does it for associations.

Tags: rails, ruby
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