Skip to Content Skip to Search

class ActiveRecord::FixtureSet

Active Record Fixtures

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

They are stored in YAML files, one file per model, which are by default placed in either <your-rails-app>/test/fixtures/ or in the test/fixtures folder under any of your application’s engines.

The location can also be changed with ActiveSupport::TestCase.fixture_paths=, once you have require "rails/test_help" in your test_helper.rb.

The fixture file ends with the .yml file extension, for example: <your-rails-app>/test/fixtures/web_sites.yml).

The format of a fixture file looks like this:

  id: 1
  name: Ruby on Rails

  id: 2
  name: Google

This fixture file includes two fixtures. Each YAML fixture (i.e. 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.


Fixtures by default are unordered. This is because the maps in YAML are unordered.

If you want ordered fixtures, use the omap YAML type. See 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.

For example:

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

Using Fixtures in Test Cases

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

By default, test_helper.rb will load all of your fixtures into your test database, so this test will succeed.

The testing environment will automatically load all the 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.

Passing in a fixture name to this dynamic method returns the fixture matching this name:

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

Passing in multiple fixture names returns all fixtures matching these names:

test "find all by name" do
  assert_equal 2, web_sites(:rubyonrails, :google).length

Passing in no arguments returns all fixtures:

test "find all" do
  assert_equal 2, web_sites.length

Passing in any fixture name that does not exist will raise StandardError:

test "find by name that does not exist" do
  assert_raise(StandardError) { web_sites(:reddit) }

If the model names conflicts with a TestCase methods, you can use the generic fixture accessor

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

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']

test "find_alt_method_2" do
  assert_equal "Ruby on Rails",

In order to use these methods to access fixtured data within your test cases, 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

Sometimes 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 fixtures to create a bunch of fixtures for load testing, like:

<% 1.upto(1000) do |i| %>
fix_<%= i %>:
  id: <%= i %>
  name: guy_<%= i %>
<% end %>

This will create 1000 very simple fixtures.

Using ERB, you can also inject dynamic values into your fixtures with inserts like <%="%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.

Helper methods defined in a fixture will not be available in other fixtures, to prevent against unwanted inter-test dependencies. Methods used by multiple fixtures should be defined in a module that is included in ActiveRecord::FixtureSet.context_class.

  • define a helper method in test_helper.rb

    module FixtureFileHelpers
      def file_sha(path)
        OpenSSL::Digest::SHA256.hexdigest('test/fixtures', path)))
    ActiveRecord::FixtureSet.context_class.include FixtureFileHelpers
  • use the helper method in a fixture

      name: kitten.png
      sha: <%= file_sha 'files/kitten.png' %>

Transactional Tests

Test cases 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_tests = true

  test "godzilla" do
    assert_not_empty Foo.all
    assert_empty Foo.all

  test "godzilla aftermath" do
    assert_not_empty Foo.all

If you preload your test database with all fixture data (probably by running bin/rails db:fixtures:load) and use transactional tests, 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 tests:

  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 Fixtures

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

There are some more advanced features available even if the id is specified:

  • Autofilled timestamp columns

  • Fixture label interpolation

  • Support for YAML defaults

Stable, Autogenerated IDs

Here, have a monkey fixture:

  id: 1
  name: George the Monkey

  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.


Let’s break out some more monkeys and pirates.

### in pirates.yml

  id: 1
  name: Reginald the Pirate
  monkey_id: 1

### in monkeys.yml

  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

  name: Reginald the Pirate
  monkey: george

### in monkeys.yml

  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

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

Can we do better? You bet!

  eater: george (Monkey)

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

has_and_belongs_to_many or has_many :through

Time to give our monkey some fruit.

### in monkeys.yml

  id: 1
  name: George the Monkey

### in fruits.yml

  id: 1
  name: apple

  id: 2
  name: orange

  id: 3
  name: grape

### in fruits_monkeys.yml

  fruit_id: 1
  monkey_id: 1

  fruit_id: 2
  monkey_id: 1

  fruit_id: 3
  monkey_id: 1

Let’s make the HABTM fixture go away.

### in monkeys.yml

  id: 1
  name: George the Monkey
  fruits: apple, orange, grape

### in fruits.yml

  name: apple

  name: orange

  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

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:

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

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

  monkey_id: <%= ActiveRecord::FixtureSet.identify(:reginald) %>
  pirate_id: <%= ActiveRecord::FixtureSet.identify(:george) %>

If the model uses UUID values for identifiers, add the :uuid argument:

ActiveRecord::FixtureSet.identify(:boaty_mcboatface, :uuid)

Support for YAML defaults

You can set and reuse defaults in your fixtures YAML file. This is the same technique used in the database.yml file to specify defaults:

  created_on: <%= 3.weeks.ago.to_fs(:db) %>

  name: Smurf

  name: Fraggle

Any fixture labeled “DEFAULTS” is safely ignored.

Besides using “DEFAULTS”, you can also specify what fixtures will be ignored by setting “ignore” in “_fixture” section.

# users.yml
    - base
  # or use "ignore: base" when there is only one fixture that needs to be ignored.

base: &base
  admin: false
  introduction: "This is a default description"

  <<: *base
  admin: true

  <<: *base

In the above example, ‘base’ will be ignored when creating fixtures. This can be used for common attributes inheriting.

Composite Primary Key Fixtures

Fixtures for composite primary key tables are fairly similar to normal tables. When using an id column, the column may be omitted as usual:

# app/models/book.rb
class Book < ApplicationRecord
  self.primary_key = [:author_id, :id]
  belongs_to :author

# books.yml
  author_id: <%= ActiveRecord::FixtureSet.identify(:lewis_carroll) %>
  title: "Alice's Adventures in Wonderland"

However, in order to support composite primary key relationships, you must use the ‘composite_identify` method:

# app/models/book_orders.rb
class BookOrder < ApplicationRecord
  self.primary_key = [:shop_id, :id]
  belongs_to :order, foreign_key: [:shop_id, :order_id]
  belongs_to :book, foreign_key: [:author_id, :book_id]

# book_orders.yml
  author: lewis_carroll
  book_id: <%= ActiveRecord::FixtureSet.composite_identify(
               :alices_adventure_in_wonderland, Book.primary_key)[:id] %>
  shop: book_store
  order_id: <%= ActiveRecord::FixtureSet.composite_identify(
               :books, Order.primary_key)[:id] %>

Configure the fixture model class

It’s possible to set the fixture’s model class directly in the YAML file. This is helpful when fixtures are loaded outside tests and set_fixture_class is not available (e.g. when running bin/rails db:fixtures:load).

  model_class: User
  name: David

Any fixtures labeled “_fixture” are safely ignored.


2**30 - 1


[R] config
[R] fixtures
[R] ignored_fixtures
[R] model_class
[R] name
[R] table_name

Public class methods

Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 578
def cache_fixtures(connection_pool, fixtures_map)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 562
def cache_for_connection_pool(connection_pool)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 570
def cached_fixtures(connection_pool, keys_to_fetch = nil)
  if keys_to_fetch

Returns a consistent, platform-independent hash representing a mapping between the label and the subcomponents of the provided composite key.


composite_identify("label", [:a, :b, :c]) # => { a: hash_1, b: hash_2, c: hash_3 }
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 635
def composite_identify(label, key)
    .with_index { |sub_key, index| (identify(label) << index) % MAX_ID }

Superclass for the evaluation contexts used by ERB fixtures.

Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 643
def context_class
  @context_class ||=
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 597
def create_fixtures(fixtures_directories, fixture_set_names, class_names = {}, config = ActiveRecord::Base)
  fixture_set_names = Array(fixture_set_names).map(&:to_s)

  connection_pool = config.connection_pool
  fixture_files_to_read = fixture_set_names.reject do |fs_name|
    fixture_is_cached?(connection_pool, fs_name)

  if fixture_files_to_read.any?
    fixtures_map = read_and_insert(
    cache_fixtures(connection_pool, fixtures_map)
  cached_fixtures(connection_pool, fixture_set_names)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 566
def fixture_is_cached?(connection_pool, table_name)

Returns a consistent, platform-independent identifier for label.

Integer identifiers are values less than 2^30. UUIDs are RFC 4122 version 5 SHA-1 hashes.

Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 621
def identify(label, column_type = :integer)
  if column_type == :uuid
    Digest::UUID.uuid_v5(Digest::UUID::OID_NAMESPACE, label.to_s)
    Zlib.crc32(label.to_s) % MAX_ID
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 591
def instantiate_all_loaded_fixtures(object, load_instances = true)
  all_loaded_fixtures.each_value do |fixture_set|
    instantiate_fixtures(object, fixture_set, load_instances)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 582
def instantiate_fixtures(object, fixture_set, load_instances = true)
  return unless load_instances
  fixture_set.each do |fixture_name, fixture|
    object.instance_variable_set "@#{fixture_name}", fixture.find
  rescue FixtureClassNotFound
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 715
def initialize(_, name, class_name, path, config = ActiveRecord::Base)
  @name     = name
  @path     = path
  @config   = config

  self.model_class = class_name
  @fixtures = read_fixture_files(path)

  @table_name = model_class&.table_name || self.class.default_fixture_table_name(name, config)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 558
def reset_cache

Public instance methods

Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 726
def [](x)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 730
def []=(k, v)
  fixtures[k] = v
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 734
def each(&block)
Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 738
def size

Returns a hash of rows to be inserted. The key is the table, the value is a list of rows to insert to that table.

Source code GitHub
# File activerecord/lib/active_record/fixtures.rb, line 744
def table_rows
  # allow specifying fixtures to be ignored by setting `ignore` in `_fixture` section
    model_class: model_class,
    fixtures: fixtures,

Definition files