Last Update: Wed Feb 20 01:30:13 -0500 2008

The NullDB Connection Adapter Plugin


NullDB is the Null Object pattern as applied to ActiveRecord database adapters. It is a database backend that translates database interactions into no-ops. Using NullDB enables you to test your model business logic - including after_save hooks - without ever touching a real database.


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

  ActiveRecord::Base.establish_connection :adapter => :nulldb

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:

  ActiveRecord::Base.establish_connection :adapter => :nulldb,
                                          :schema  => foo/myschema.rb

NullDB comes with RSpec integration. To replace the database with NullDB in all of your specs, put the following in your spec/spec_helper:

  require 'nulldb_rspec'
  include NullDB::RSpec::NullifiedDatabase

Or if you just want to use NullDB in a specific spec context, you can include the same module inside a context:

  require 'nulldb_rspec'

  describe Employee, "with access to the database" do
    fixtures :employees
    # ...

  describe Employee, "with NullDB" do
    include NullDB::RSpec::NullifiedDatabase
    # ...

NullDB::Rspec provides some custom matcher support for verifying expectations about interactions with the database:

  describe Employee do
    include NullDB::RSpec::NullifiedDatabase

    it "should cause an insert statement to be executed" do
      Employee.connection.should have_executed(:insert)

UnitRecord-style verification that no database calls have been made at all can be achieved by using the special +:anything+ symbol:

  describe "stuff that shouldn't touch the database" do
    after :each do
      Employee.connection.should_not have_executed(:anything)
    # ...

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

    adapter: nulldb

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.


There are a number of advantages to writing unit tests that never touch the database. The biggest is probably speed of execution - unit tests must be fast for test-driven development to be practical. Another is separation of concerns: unit tests should be exercising only the business logic contained in your models, not ActiveRecord. For more on why testing-sans-database is a god idea, see:

NullDB is one way to separate your unit tests from the database. It 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. Test fixtures won‘t work either, for obvious reasons.
  • 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 <>



 * Version 0.0.1 (2007-02-18)
   - Initial Release


See the LICENSE file for licensing information.