Fixtures are a way of organizing data that you want to test against; in short, sample data.

Fixture formats

Fixtures come in 3 flavors:

  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, human-readable format. It ships with Ruby 1.8.1+.

Unlike single-file fixtures, YAML fixtures are stored in a single file per model, which are placed in the directory appointed by ActiveSupport::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.

Note that YAML fixtures are unordered. If you want ordered fixtures, use the omap YAML type. See http://yaml.org/type/omap.html for the specification. You will need ordered fixtures when you have foreign key constraints on keys in the same table. This is commonly needed for tree structures. Example:

   --- !omap
   - parent:
       id:         1
       parent_id:  NULL
       title:      Parent
   - child:
       id:         2
       parent_id:  1
       title:      Child

CSV fixtures

Fixtures can also be kept in the Comma Separated Value (CSV) 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 fixture 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 ActiveSupport::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 in testcases

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 let’s take a look at a sample unit test:

  require 'test_helper'

  class WebSiteTest < ActiveSupport::TestCase
    test "web_site_count" do
      assert_equal 2, WebSite.count
    end
  end

By default, the test_helper module will load all of your fixtures into your test database, so this test will succeed. The testing environment will automatically load the all fixtures into the database before each test. To ensure consistent data, the environment deletes the fixtures before running the load.

In addition to being available in the database, the fixture’s data may also be accessed by using a special dynamic method, which has the same name as the model, and accepts the name of the fixture to instantiate:

  test "find" do
    assert_equal "Ruby on Rails", web_sites(:rubyonrails).name
  end

Alternatively, you may enable auto-instantiation of the fixture data. For instance, take the following tests:

  test "find_alt_method_1" do
    assert_equal "Ruby on Rails", @web_sites['rubyonrails']['name']
  end

  test "find_alt_method_2" do
    assert_equal "Ruby on Rails", @rubyonrails.news
  end

In order to use these methods to access fixtured data within your testcases, you must specify one of the following in your ActiveSupport::TestCase-derived class:

• to fully enable instantiated fixtures (enable alternate methods #1 and #2 above)

    self.use_instantiated_fixtures = true
  

• create only the hash for the fixtures, do not ‘find’ each instance (enable alternate method #1 only)

    self.use_instantiated_fixtures = :no_instances
  

Using either of these alternate methods incurs a performance hit, as the fixtured data must be fully traversed in the database to create the fixture hash and/or instance variables. This is expensive for large sets of fixtured data.

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.

  class FooTest < ActiveSupport::TestCase
    self.use_transactional_fixtures = true

    test "godzilla" do
      assert !Foo.find(:all).empty?
      Foo.destroy_all
      assert Foo.find(:all).empty?
    end

    test "godzilla aftermath" do
      assert !Foo.find(:all).empty?
    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.

In order to use instantiated fixtures with preloaded data, set self.pre_loaded_fixtures to true. This will provide access to fixture data for every table that has been loaded through fixtures (depending on the value of use_instantiated_fixtures)

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.

Advanced YAML Fixtures

YAML fixtures that don’t specify an ID get some extra features:

• Stable, autogenerated IDs
• Label references for associations (belongs_to, has_one, has_many)
• HABTM associations as inline lists
• Autofilled timestamp columns
• Fixture label interpolation
• Support for YAML defaults

Stable, autogenerated IDs

Here, have a monkey fixture:

  george:
    id: 1
    name: George the Monkey

  reginald:
    id: 2
    name: Reginald the Pirate

Each of these fixtures has two unique identifiers: one for the database and one for the humans. Why don’t we generate the primary key instead? Hashing each fixture’s label yields a consistent ID:

  george: # generated id: 503576764
    name: George the Monkey

  reginald: # generated id: 324201669
    name: Reginald the Pirate

Active Record looks at the fixture’s model class, discovers the correct primary key, and generates it right before inserting the fixture into the database.

The generated ID for a given label is constant, so we can discover any fixture’s ID without loading anything, as long as we know the label.

Label references for associations (belongs_to, has_one, has_many)

Specifying foreign keys in fixtures can be very fragile, not to mention difficult to read. Since Active Record can figure out the ID of any fixture from its label, you can specify FK’s by label instead of ID.

belongs_to

Let’s break out some more monkeys and pirates.

  ### in pirates.yml

  reginald:
    id: 1
    name: Reginald the Pirate
    monkey_id: 1

  ### in monkeys.yml

  george:
    id: 1
    name: George the Monkey
    pirate_id: 1

Add a few more monkeys and pirates and break this into multiple files, and it gets pretty hard to keep track of what’s going on. Let’s use labels instead of IDs:

  ### in pirates.yml

  reginald:
    name: Reginald the Pirate
    monkey: george

  ### in monkeys.yml

  george:
    name: George the Monkey
    pirate: reginald

Pow! All is made clear. Active Record reflects on the fixture’s model class, finds all the belongs_to associations, and allows you to specify a target label for the association (monkey: george) rather than a target id for the FK (monkey_id: 1).

Polymorphic belongs_to

Supporting polymorphic relationships is a little bit more complicated, since Active Record needs to know what type your association is pointing at. Something like this should look familiar:

  ### in fruit.rb

  belongs_to :eater, :polymorphic => true

  ### in fruits.yml

  apple:
    id: 1
    name: apple
    eater_id: 1
    eater_type: Monkey

Can we do better? You bet!

  apple:
    eater: george (Monkey)

Just provide the polymorphic target type and Active Record will take care of the rest.

has_and_belongs_to_many

Time to give our monkey some fruit.

  ### in monkeys.yml

  george:
    id: 1
    name: George the Monkey
    pirate_id: 1

  ### in fruits.yml

  apple:
    id: 1
    name: apple

  orange:
    id: 2
    name: orange

  grape:
    id: 3
    name: grape

  ### in fruits_monkeys.yml

  apple_george:
    fruit_id: 1
    monkey_id: 1

  orange_george:
    fruit_id: 2
    monkey_id: 1

  grape_george:
    fruit_id: 3
    monkey_id: 1

Let’s make the HABTM fixture go away.

  ### in monkeys.yml

  george:
    name: George the Monkey
    pirate: reginald
    fruits: apple, orange, grape

  ### in fruits.yml

  apple:
    name: apple

  orange:
    name: orange

  grape:
    name: grape

Zap! No more fruits_monkeys.yml file. We’ve specified the list of fruits on George’s fixture, but we could’ve just as easily specified a list of monkeys on each fruit. As with belongs_to, Active Record reflects on the fixture’s model class and discovers the has_and_belongs_to_many associations.

Autofilled timestamp columns

If your table/model specifies any of Active Record's standard timestamp columns (created_at, created_on, updated_at, updated_on), they will automatically be set to Time.now.

If you’ve set specific values, they’ll be left alone.

Fixture label interpolation

The label of the current fixture is always available as a column value:

  geeksomnia:
    name: Geeksomnia's Account
    subdomain: $LABEL

Also, sometimes (like when porting older join table fixtures) you’ll need to be able to get ahold of the identifier for a given label. ERB to the rescue:

  george_reginald:
    monkey_id: <%= Fixtures.identify(:reginald) %>
    pirate_id: <%= Fixtures.identify(:george) %>

Support for YAML defaults

You probably already know how to use YAML to set and reuse defaults in your database.yml file. You can use the same technique in your fixtures:

  DEFAULTS: &DEFAULTS
    created_on: <%= 3.weeks.ago.to_s(:db) %>

  first:
    name: Smurf
    <<: *DEFAULTS

  second:
    name: Fraggle
    <<: *DEFAULTS

Any fixture labeled "DEFAULTS" is safely ignored.