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YAML's behavior in this guide. [ci skip]

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loading fixtures to the database.

Reference #17542

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.

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Partially revert https://github.com/rails/rails/commit/c31e638a37946a120f6dbf207127d36beea5ab85.

This has changed recently and so rake test:all and rake
test:all:db are deprecated.

Add missing testing tasks and also improve upon tasks descriptions

name keywords

Renames `rake test:all` to `rake test` by changing old `rake test:run` to previous version of `rake test:all`.  Removes old definition of `rake test`. Also renames `rake test:all:db` to `rake test:db` and deprecates `rake test:all` & `rake test:all:db`

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.

- Integration test example is now using 'articles' name
  for both path and assigns key.

* Cover lower level job testing (assertions about the execution of the
  job itself).
* Add a table with the custom assertions provided by Active Job's
  `TestHelper` module.
* Add an example testing job queuing inside a model

shows the class that the test must extend in order to have the assert_routing method available

Currently there is no implicit conversion for Mail::Body to string
therefore .to_s must be applied. This is exampled in 10.2.2.

When you manually specify `id:` attribute in a fixture Rails' built in association assignment feature will no longer work. http://stackoverflow.com/questions/23985675/incorrect-association-in-fixtures

This commit raises awareness of this behavior and encourages developers to look directly at the fixture documentation.

The fixtures API docs are really comprehensive. We should either be driving people there, or perhaps making fixtures its own guide.

The Guide references `test/unit`, which has been replaced by `Minitest` in
all versions of Ruby supported by Rails. The following updates were
performed:

* The superclass of `ActiveSupport::TestCase` is no longer
  `MiniTest::Unit::TestCase`, but `Minitest::Test` [1].

* The preferred spelling is `Minitest`, not `MiniTest` [2].

* For a method to be a test in Minitest, its name must start with `test_`,
  not only `test` [3].

* Explanations about `test/unit` in Ruby 1.8 were removed in favor of more
  up-to-date information on `Minitest`.

[1]: https://github.com/rails/rails/blob/6c2b569660c9216595d02ce6412e3bdcc7293317/activesupport/lib/active_support/test_case.rb#L18
[2]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest.rb#L9
[3]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest/test.rb#L62

I don't know when this changed, checked back up to 3.2 and integration
tests instantiate fixtures just fine.

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