Announcing NullDB 0.0.1

I spent the afternoon coding an alternative to the ARBS and UnitRecord database-elimination plugins.  Definitely scratching a personal itch, as I had a project where I wanted to use one of the above-mentioned libraries but i just couldn’t seem to get them to work.  I’m releasing it in hopes others will find it useful too.

Here’s the text of the README:


NullDB is a Rails database connection adapter that interprets common database operations as no-ops. It is the Null Object pattern as applied to database adapters.


Once installed, NullDB can be used much like any other ActiveRecord database adapter:

NullDB needs to know where you keep your schema file in order to reflect table metadata. By default it looks in RAILS_ROOT/db/schema.rb. You can override that by setting the schema option:

There is a helper method included for configuring RSpec sessions to use NullDB. Just put the following in your spec/spec_helper.rb:

You can also experiment with putting NullDB in your database.yml:

However, due to the way Rails hard-codes specific database adapters into its standard Rake tasks, you may find that this generates unexpected and difficult-to-debug behavior. Workarounds for this are under development.


NullDB is intended to assist in writing fast database-independant unit tests for ActiveRecord classes. For why you would want to test your models without the database, see:

NullDB was inspired by the ARBS and UnitRecord libraries. It differs from them in a couple of ways:

  1. It works. At the time of writing both ARBS and UnitRecord were not working for me out of the box with Rails 2.0.
  2. It avoids monkey-patching as much as possible. Rather than re-wiring the secret inner workings of ActiveRecord (and thus being tightly coupled to those inner workings), NullDB implements the same [semi-]well-documented public interface that the other standard database adapters, like MySQL and SQLServer, implement.
  3. UnitRecord takes the approach of eliminating database interaction in tests by turning almost every database interaction into an exception. NullDB recognizes that ActiveRecord objects typically can‘t take two steps without consulting the database, so instead it turns database interactions into no-ops.

One concrete advantage of this null-object pattern design is that it is possible with NullDB to test after_save hooks. With NullDB, you can call +save+ and all of the usual callbacks will be called – but nothing will be saved.


  • It is not an in-memory database. Finds will not work. Neither will reload, currently.
  • It has only the most rudimentery schema/migration support. Complex migrations will probably break it.
  • Lots of other things probably don‘t work. Patches welcome!


NullDB was written by Avdi Grimm <>