Testing ActiveRecord Transactions

Posted by Luke Francl
on Wednesday, March 28

ActiveRecord allows you to start transactions that will be rolled back in the event of an error.

A good example is importing records from a CSV file. If you want the entire import to roll back if any of the rows fail to import, you could write your code like this:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

def import_csv
  csv_file = params[:csv_file]
  
  begin
    Record.transaction do 
      fastercsv = FasterCSV.new( csv_file )
      while row = fastercsv.readline
        foo, bar = row
        Record.create!( :foo => foo, :bar => bar )
      end
    end
    redirect_to success_action_path
  rescue 
    # do something with the error
    flash[:error] = "CSV import failed"
    redirect_to import_path
  end
end

The Record.create! call will throw an ActiveRecord::InvalidRecord error if one of the rows can’t be saved. Then the rescue block catches the error and reports it to the user instead of showing them an ugly 500 error (or, worse, a corrupted import).

However, this doesn’t play nicely with your tests.

You’d like to do something like this:

1
2
3
4
5

def test_import_csv_failure
  assert_no_difference Record :count do 
    post :import_csv, :csv_file => fixture_file_upload('files/invalid.csv')
  end
end

But this won’t work, because running the test starts a transaction, and ActiveRecord doesn’t support nested transactions. There’s been a patch open on this problem for 9 months, but no action has been taken.

I was able to work around the problem by turning off transactional fixtures for the entire test case class.

1
2
3
4
5
6
7
8
9

class MyTest < Test::Unit::TestCase
  self.use_transactional_fixtures = false

  def test_import_csv_failure
    assert_no_difference Record :count do 
      post :import_csv, :csv_file => fixture_file_upload('files/invalid.csv')
    end
  end
end

This makes the test run slower, but now it passes. If you’re feeling adventurous, you can install the ActiveRecord nested transactions plugin.

Lots of people have hit this problem. Jerry Kuch blogged about it in January 2006 and ticket 5457 was filed back in June. But hopefully this post will help someone else figure out the problem.