If you use transactional_fixtures or the 
  database_cleaner gem
  
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 with strategy :transaction, after_commit callbacks will not be fired in your tests.
Rails 5+
Rails 5 has a fix Show archive.org snapshot for this issue and no further action is needed.
Rails 3, Rails 4
Add the 
  gem test_after_commit
  
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 to your test group in the Gemfile and you are done. You don't need to change the database strategy to deletion (which might lead to longer test runs).
Note:
Most often a after_commit is the only right choice. Sending emails, performing async sidekiq workers and many things more are kind of broken with an after_save callback.
If you replace an after_save callback with an after_commit callback this list might help you to fix your code afterwards:
- 
attribute_changed?will return true in theafter_savecallback and false in theafter_commitcallback. Use something likeprevious_changes.key?(:attribute)instead.