Added missing `file` delivery method to the Configuration Guide

Ensure AR #second, #third, etc. finders work through associations

This commit fixes two regressions introduced in cafe31a0 where
newly created finder methods #second, #third, #forth, and #fifth
caused a NoMethodError error on reload associations and where we
were pulling the wrong element out of cached associations.

Examples:

  some_book.authors.reload.second

  # Before
  # => NoMethodError: undefined method 'first' for nil:NilClass

  # After
  # => #<Author id: 2, name: "Sally Second", ...>

  some_book.first.authors.first
  some_book.first.authors.second

  # Before
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 1, name: "Freddy First", ...>

  # After
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 2, name: "Sally Second", ...>

Fixes #13783.

Implement the Dirty API with the Enum feature correctly.

Conflicts:
	activerecord/CHANGELOG.md

This will make simpler to compare if the values changed in the
save_changed_attribute method.

Add missing information for `id` field when used with `select` [ci skip]

Since Rails 4.0, we add an ORDER BY in the `first` method to ensure consistent
results among different database engines. But for singular associations this
behavior is not needed since we will have one record to return. As this
ORDER BY option can lead some performance issues we are removing it for singular
associations accessors.

Fixes #12623.

This fixes a bug where `select(:id)` combined with `joins()` raised:

```
ActiveRecord::StatementInvalid: SQLite3::SQLException: ambiguous column name: id:
SELECT  id, authors.author_address_id
FROM "posts"
INNER JOIN "authors"
ON "authors"."id" = "posts"."author_id"
ORDER BY posts.id LIMIT 3
```

The `select_values` are still String and Symbols because other parts (mainly calculations.rb)
rely on that fact.

/cc @tenderlove

To make this possible we have to override the save_changed_attribute
hook.

This will make easier to hook on this feature to customize the behavior

setup Bundler in engines `bin/rails` stub.

Fail early with "Primary key not included in the custom select clause" i...

Before this patch find_in_batches raises this error only on second iteration. So you will know about the problem only when you get the batch size threshold.

With the introduction of `#second` method and friends, we added an
offsets hash which replaced the @first variable, so removing it from the
reset method to avoid creating an unused variable now.

Introduced in bc625080.

Before this patch after the changes are applied the changes can be only
accessed using string keys, but before symbols are also accepted.

After this change every state of the model will be consistent.

This variable is internal and should not be exposed to end users

Ensure #second (and others) acts like #first AR finder

This commit bring the famous ordinal Array instance methods defined
in ActiveSupport into ActiveRecord as fully-fledged finders.

These finders ensure a default ascending order of the table's primary
key, and utilize the OFFSET SQL verb to locate the user's desired
record. If an offset is defined in the query, calling #second adds
to the offset to get the actual desired record.

Fixes #13743.

The PR #13767 added link to the upgrading guide to explain
details about activerecord_deprecated-finders. However the link target
features a stack of changes not releated at all. Also the relevant details
are not very informative.

I think we better link to the README so people can see what the gem is about
and how to use it.

/cc @chancancode @zzak

The file is config/secrets.yml, not config/initializers/secrets.yml.

Add link to upgrade guide for full list of deprecated finders [ci skip]

Fix a typo that says you application [ci skip]

Fix for incorrect link that is pointing to Rails blog [ci skip]

Update Error Message when secrets.secret_key_base isn't given