Since #18937 `belongs_to` associations receive a setting to determine if
it should be or not treated as `required` by default.
While the tests were still passing, it was not evident that the
"default" behaviour for `required` could change in fuction of a setting,
that is set by default for fresh Rails5 apps, but not for upgraded
apps.
This commit try to relate them to make it clear what is the behaviour
expected when the setting is set as `true` or not set.
I’m renaming all instances of `use_transcational_fixtures` to
`use_transactional_tests` and “transactional fixtures” to
“transactional tests”.
I’m deprecating `use_transactional_fixtures=`. So anyone who is
explicitly setting this will get a warning telling them to use
`use_transactional_tests=` instead.
I’m maintaining backwards compatibility—both forms will work.
`use_transactional_tests` will check to see if
`use_transactional_fixtures` is set and use that, otherwise it will use
itself. But because `use_transactional_tests` is a class attribute
(created with `class_attribute`) this requires a little bit of hoop
jumping. The writer method that `class_attribute` generates defines a
new reader method that return the value being set. Which means we can’t
set the default of `true` using `use_transactional_tests=` as was done
previously because that won’t take into account anyone using
`use_transactional_fixtures`. Instead I defined the reader method
manually and it checks `use_transactional_fixtures`. If it was set then
it should be used, otherwise it should return the default, which is
`true`. If someone uses `use_transactional_tests=` then it will
overwrite the backwards-compatible method with whatever they set.
In addition to defining the association, a `required` association will
also have its presence validated.
Before:
```ruby
belongs_to :account
validates_presence_of :account
```
After:
```ruby
belongs_to :account, required: true
```
This helps to draw a distinction between types of validations, since
validations on associations are generally for data integrity purposes,
and aren't usually set through form inputs.
