Updated following multiple comments requesting examples and questions on where to put AR::Base subclass tests.

Everyone (who reads this blog anyway) knows that you should not cross boundaries while unit testing. Unfortunately, Ruby on Rails seems to believe otherwise. This is evident by the fact that the test:units rake task has the pre-requisite db:test:prepare. Additionally, if you use script/generate to create a model, it creates a [model].yml fixture file and a unit test that includes a call to the fixtures class method. Rails may be opinionated, but that doesn't mean I have to agree with it.

With a minor modification you can tell Rails not to run the db:test:prepare task. You should also create a new test helper that doesn't load the additional frameworks that you will not need. I found some of the code for this from reading a great book, Rails Recipes, by Chad Fowler.

You'll need to add a .rake file to /lib/tasks. The file contains one line:

Rake::Task[:'test:units'].prerequisites.clear

Additionally, you'll need to create a new helper file in /test. I named my file unit_test_helper.rb, but the file name is your choice.

ENV["RAILS_ENV"] = "test" 
require File.expand_path(File.dirname(__FILE__) + "/../config/environment") 
require 'application' 
require 'test/unit' 
require 'action_controller/test_process' 
require 'breakpoint'

class UnitTest
  def self.TestCase
    class << ActiveRecord::Base
      def connection
        raise InvalidActionError, 'You cannot access the database from a unit test', caller
      end
    end
    Test::Unit::TestCase
  end
end

class InvalidActionError < StandardError
end

As you can see, the unit_test_helper.rb only requires what is necessary; however, it also changes ActiveRecord::Base to throw an error if you attempt to access the connection from a unit test.

I included this test in my codebase to ensure expected behavior.

require File.dirname(__FILE__) + '/../unit_test_helper'

class AttemptToAccessDbThrowsExceptionTest < UnitTest.TestCase
  def test_calling_the_db_causes_a_failure
    assert_raise(InvalidActionError) { ActiveRecord::Base.connection }
  end
end

Update:
We have been using this style of testing for several months now and have over 130 tests at this point and our tests still run in less than a second.

This decision does carry some trade-offs though. First of all, you cannot test your ActiveRecord::Base subclasses in your unit tests. I'm comfortable with this since the AR::Base classes hit the database, thus making them perfect candidates for functional tests.

Also, if you need to use a AR::Base class as a dependency for another class, you will need to mock or stub the AR::Base class. This generally requires using Dependency Injection or a framework such as Stubba.

For an example of what our unit tests look like here are some tests and the classes that the tests cover.

require File.dirname(__FILE__) + '/../../../unit_test_helper'

class SelectTest < Test::Unit::TestCase 
  def test_select_with_single_column
    assert_equal 'select foo', Select[:foo].to_sql
  end

  def test_select_with_multiple_columns
    assert_equal 'select foo, bar', Select[:foo, :bar].to_sql
  end
  
  def test_date_time_literals_quoted
    date = DateTime.new(2006, 1, 20, 13, 30, 54)
    assert_equal "select to_timestamp('2006-01-20 13:30:54', 'YYYY-MM-DD HH24:MI:SS')", Select[date].to_sql
  end
  
  def test_select_with_symbol_and_literal_columns
    assert_equal "select symbol, 'literal'", Select[:symbol, 'literal'].to_sql
  end
  
  def test_select_with_single_table
    assert_equal 'select foo from foo', Select[:foo].from[:foo].to_sql
  end
    
  def test_select_with_multiple_tables
    assert_equal 'select column from bar, foo',
                 Select[:column].from[:foo, :bar].to_sql
  end
end 

require File.dirname(__FILE__) + '/../../unit_test_helper'

class TimeTest < Test::Unit::TestCase 
  def test_to_sql_gives_quoted
    t = Time.parse('2006/05/01')
    assert_equal "to_timestamp('2006-05-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS')", t.to_sql
  end
  
  def test_to_pretty_datetime
    d = Time.parse("05/10/2006")
    assert_equal "05-10-2006 12:00 AM", d.to_pretty_time
  end
end 

class Select < SqlStatement
  class << self
    def [](*columns)
      unless columns.select { |column| column.nil? }.empty?
        raise "Empty Column in #{columns.inspect}"
      end
      self.new("select #{columns.collect{ |column| column.to_sql }.join(', ')}")
    end
  end
  
  def from
    @to_sql += " from "
    self
  end
  
  def [](*table_names)
    @to_sql += table_names.collect{ |table| table.to_s }.sort.join(', ')
    self
  end
  
end

class Time
  def to_sql
    "to_timestamp('" + formatted + "', 'YYYY-MM-DD HH24:MI:SS')"
  end
  
  def to_pretty_time
    self.strftime("%m-%d-%Y %I:%M %p")
  end

  private 
  
  def formatted
    year.to_s + "-" + month.pad + "-" + day.pad + " " + hour.pad + ":" + min.pad + ":" + sec.pad
  end
end