This method is the source of many useless allocations:

```
allocated objects by location
-----------------------------------
     39568  ~/gems/rails-25a5b296/activesupport/lib/active_support/callbacks.rb:846

allocated memory by location
-----------------------------------
  17.11 MB  ~/gems/rails-25a5b296/activesupport/lib/active_support/callbacks.rb:846

retained memory by location
-----------------------------------
   1.03 MB  ~/gems/rails-25a5b296/activesupport/lib/active_support/callbacks.rb:846
```