Fixtures are a way of organizing data that you want to test against; in short, sample data. They come in 3 flavours:

  1.  YAML fixtures
  2.  CSV fixtures
  3.  Single-file fixtures

YAML fixtures

This type of fixture is in YAML format and the preferred default. YAML is a file format which describes data structures in a non-verbose, humanly-readable format. It ships with Ruby 1.8.1+.

Unlike single-file fixtures, YAML fixtures are stored in a single file per model, which is place in the directory appointed by Test::Unit::TestCase.fixture_path=(path) (this is automatically configured for Rails, so you can just put your files in <your-rails-app>/test/fixtures/). The fixture file ends with the .yml file extension (Rails example: "<your-rails-app>/test/fixtures/web_sites.yml"). The format of a YAML fixture file looks like this:

  rubyonrails:
    id: 1
    name: Ruby on Rails
    url: http://www.rubyonrails.org

  google:
    id: 2
    name: Google
    url: http://www.google.com

This YAML fixture file includes two fixtures. Each YAML fixture (ie. record) is given a name and is followed by an indented list of key/value pairs in the "key: value" format. Records are separated by a blank line for your viewing pleasure.

CSV fixtures

Fixtures can also be kept in the Comma Separated Value format. Akin to YAML fixtures, CSV fixtures are stored in a single file, but, instead end with the .csv file extension (Rails example: "<your-rails-app>/test/fixtures/web_sites.csv")

The format of this type of fixture file is much more compact than the others, but also a little harder to read by us humans. The first line of the CSV file is a comma-separated list of field names. The rest of the file is then comprised of the actual data (1 per line). Here’s an example:

  id, name, url
  1, Ruby On Rails, http://www.rubyonrails.org
  2, Google, http://www.google.com

Should you have a piece of data with a comma character in it, you can place double quotes around that value. If you need to use a double quote character, you must escape it with another double quote.

Another unique attribute of the CSV fixture is that it has no fixture name like the other two formats. Instead, the fixture names are automatically generated by deriving the class name of the fixture file and adding an incrementing number to the end. In our example, the 1st fixture would be called "web_site_1" and the 2nd one would be called "web_site_2".

Most databases and spreadsheets support exporting to CSV format, so this is a great format for you to choose if you have existing data somewhere already.

Single-file fixtures

This type of fixtures was the original format for Active Record that has since been deprecated in favor of the YAML and CSV formats. Fixtures for this format are created by placing text files in a sub-directory (with the name of the model) to the directory appointed by Test::Unit::TestCase.fixture_path=(path) (this is automatically configured for Rails, so you can just put your files in <your-rails-app>/test/fixtures/<your-model-name>/ — like <your-rails-app>/test/fixtures/web_sites/ for the WebSite model).

Each text file placed in this directory represents a "record". Usually these types of fixtures are named without extensions, but if you are on a Windows machine, you might consider adding .txt as the extension. Here’s what the above example might look like:

  web_sites/google
  web_sites/yahoo.txt
  web_sites/ruby-on-rails

The file format of a standard fixture is simple. Each line is a property (or column in db speak) and has the syntax of "name => value". Here’s an example of the ruby-on-rails fixture above:

  id => 1
  name => Ruby on Rails
  url => http://www.rubyonrails.org

Using Fixtures

Since fixtures are a testing construct, we use them in our unit and functional tests. There are two ways to use the fixtures, but first lets take a look at a sample unit test found:

  require 'web_site'

  class WebSiteTest < Test::Unit::TestCase
    def test_web_site_count
      assert_equal 2, WebSite.count
    end
  end

As it stands, unless we pre-load the web_site table in our database with two records, this test will fail. Here’s the easiest way to add fixtures to the database:

  ...
  class WebSiteTest < Test::Unit::TestCase
    fixtures :web_sites # add more by separating the symbols with commas
  ...

By adding a "fixtures" method to the test case and passing it a list of symbols (only one is shown here tho), we trigger the testing environment to automatically load the appropriate fixtures into the database before each test, and automatically delete them after each test.

In addition to being available in the database, the fixtures are also loaded into a hash stored in an instance variable of the test case. It is named after the symbol… so, in our example, there would be a hash available called @web_sites. This is where the "fixture name" comes into play.

On top of that, each record is automatically "found" (using Model.find(id)) and placed in the instance variable of its name. So for the YAML fixtures, we’d get @rubyonrails and @google, which could be interrogated using regular Active Record semantics:

  # test if the object created from the fixture data has the same attributes as the data itself
  def test_find
    assert_equal @web_sites["rubyonrails"]["name"], @rubyonrails.name
  end

As seen above, the data hash created from the YAML fixtures would have @web_sites["rubyonrails"]["url"] return "www.rubyonrails.org" and @web_sites["google"]["name"] would return "Google". The same fixtures, but loaded from a CSV fixture file would be accessible via @web_sites["web_site_1"]["name"] == "Ruby on Rails" and have the individual fixtures available as instance variables @web_site_1 and @web_site_2.

Dynamic fixtures with ERb

Some times you don’t care about the content of the fixtures as much as you care about the volume. In these cases, you can mix ERb in with your YAML or CSV fixtures to create a bunch of fixtures for load testing, like:

<% for i in 1..1000 %> fix_<%= i %>:

  id: <%= i %>
  name: guy_<%= 1 %>

<% end %>

This will create 1000 very simple YAML fixtures.

Using ERb, you can also inject dynamic values into your fixtures with inserts like <%= Date.today.strftime("%Y-%m-%d") %>. This is however a feature to be used with some caution. The point of fixtures are that they’re stable units of predictable sample data. If you feel that you need to inject dynamic values, then perhaps you should reexamine whether your application is properly testable. Hence, dynamic values in fixtures are to be considered a code smell.

Transactional fixtures

TestCases can use begin+rollback to isolate their changes to the database instead of having to delete+insert for every test case. They can also turn off auto-instantiation of fixture data since the feature is costly and often unused.

  class FooTest < Test::Unit::TestCase
    self.use_transactional_fixtures = true
    self.use_instantiated_fixtures = false

    fixtures :foos

    def test_godzilla
      assert !Foo.find_all.emtpy?
      Foo.destroy_all
      assert Foo.find_all.emtpy?
    end

    def test_godzilla_aftermath
      assert !Foo.find_all.emtpy?
    end
  end

If you preload your test database with all fixture data (probably in the Rakefile task) and use transactional fixtures, then you may omit all fixtures declarations in your test cases since all the data’s already there and every case rolls back its changes.

When not to use transactional fixtures:

  1. You're testing whether a transaction works correctly. Nested transactions don't commit until all parent transactions commit,
     particularly, the fixtures transaction which is begun in setup and rolled back in teardown. Thus, you won't be able to verify
     the results of your transaction until Active Record supports nested transactions or savepoints (in progress.)
  2. Your database does not support transactions. Every Active Record database supports transactions except MySQL MyISAM.
     Use InnoDB, MaxDB, or NDB instead.