Most existing tests expects `connection_handlers` has only one default
handler, but the test added at #34779 dirties that.

We need to reset `connection_handlers` to default in that case.

Closes #35471.

Fix Getting Started guide to state the correct number of files created by the controller generator

[ci skip]

Fix typo in test name

In e8546aba, the Getting Started guide
was updated to reflect the removal of CoffeeScript stubs being generated
by the controller generator. However, the comment above the table of
generated files wasn't updated, so it incorrectly remained "This creates
five files and one empty directory". This fixes it to now say "This
creates four files and one empty directory".

Copy the forking isolated test runner from railties

* Add `ActiveRecord::Base.connection.truncate` for SQLite3 adapter.

SQLite doesn't support `TRUNCATE TABLE`, but SQLite3 adapter can support
`ActiveRecord::Base.connection.truncate` by using `DELETE FROM`.

`DELETE` without `WHERE` uses "The Truncate Optimization",
see https://www.sqlite.org/lang_delete.html.

* Add `rails db:seed:replant` that truncates database tables and loads the seeds

Closes #34765

All the tests have a substantial chunk of identical setup effort (in
cases/helper.rb); this makes isolated tests run much faster, without any
change to the variety of how we load files.

Fix typo in CHANGELOG.md [ci skip]

Deprecate mismatched collation comparison for uniquness validator

In MySQL, the default collation is case insensitive. Since the
uniqueness validator enforces case sensitive comparison by default, it
frequently causes mismatched collation issues (performance, weird
behavior, etc) to MySQL users.

https://grosser.it/2009/12/11/validates_uniqness_of-mysql-slow/
https://github.com/rails/rails/issues/1399
https://github.com/rails/rails/pull/13465
https://github.com/gitlabhq/gitlabhq/commit/c1dddf8c7d947691729f6d64a8ea768b5c915855
https://github.com/huginn/huginn/pull/1330#discussion_r55152573

I'd like to deprecate the implicit default enforcing since I frequently
experienced the problems in code reviews.

Note that this change has no effect to sqlite3, postgresql, and
oracle-enhanced adapters which are implemented as case sensitive by
default, only affect to mysql2 adapter (I can take a work if sqlserver
adapter will support Rails 6.0).

Move changelog entry about `reselect` method to the section of the next release [ci skip]

Fixes https://github.com/rails/rails/pull/33611#discussion_r261549790
Related to https://3.basecamp.com/3076981/buckets/24956/chats/12416418@1631552581

[ci skip]

Ref: https://travis-ci.org/rails/rails/jobs/501220262#L1194

Similarly to https://github.com/rails/webpacker/pull/1976

Fixed reselect throwing NoMethodError on ActiveRecord.

Remove unnecessary `current_adapter?(:OracleAdapter)` for index length

Follow up #35455, there are two more test cases unnecessary `if current_adapter?(:OracleAdapter)`

```ruby
$ ARCONN=oracle bin/test test/cases/associations/eager_test.rb -n test_include_has_many_using_primary_key
Using oracle
Run options: -n test_include_has_many_using_primary_key --seed 62842

.

Finished in 50.280024s, 0.0199 runs/s, 0.0398 assertions/s.
1 runs, 2 assertions, 0 failures, 0 errors, 0 skips
$
```

```
$ ARCONN=oracle bin/test test/cases/migration/index_test.rb -n test_add_index
Using oracle
Run options: -n test_add_index --seed 52034

.

Finished in 13.152620s, 0.0760 runs/s, 0.0000 assertions/s.
1 runs, 0 assertions, 0 failures, 0 errors, 0 skips
$
```

Oracle database can run `delete`  with `order by` and`fetch first n rows only`

Rails `DateTime` type is mapped to Oracle `TIMESTAMP` since Rails 5.0

Since https://github.com/rails/arel/pull/337 Oracle adapter uses better
top N query using `fetch first n rows only`, which can remove this
unless condition.

* This commit passes with Oracle database
```ruby
$ ARCONN=oracle bin/test test/cases/relation/delete_all_test.rb -n test_delete_all_with_order_and_limit_deletes_subset_only
Using oracle
Run options: -n test_delete_all_with_order_and_limit_deletes_subset_only --seed 1081

.

Finished in 8.068626s, 0.1239 runs/s, 0.6197 assertions/s.
1 runs, 5 assertions, 0 failures, 0 errors, 0 skips
$
```

* SQL statement includes `ORDER BY` and `FETCH FIRST n ROWS ONLY`

```sql
  Post Destroy (12.5ms)  DELETE FROM "POSTS" WHERE "POSTS"."ID" IN (SELECT "POSTS"."ID" FROM "POSTS" WHERE "POSTS"."AUTHOR_ID" = :a1 ORDER BY "POSTS"."ID" ASC FETCH FIRST :a2 ROWS ONLY)  [["author_id", 1], ["LIMIT", 1]]
```

Kind of reverting https://github.com/rails/rails/commit/3a1cbc5c3b3bcb2de4be6e4469bb87b99759dc59

```
From: test/cases/date_test.rb @ line 26 :

    21:
    22:     invalid_dates = [[2007, 11, 31], [1993, 2, 29], [2007, 2, 29]]
    23:
    24:     valid_dates.each do |date_src|
    25:       topic = Topic.new("last_read(1i)" => date_src[0].to_s, "last_read(2i)" => date_src[1].to_s, "last_read(3i)" => date_src[2].to_s)
 => 26:       binding.irb
    27:       assert_equal(topic.last_read, Date.new(*date_src))
    28:     end
    29:
    30:     invalid_dates.each do |date_src|
    31:       assert_nothing_raised do

irb(#<DateTest:0x0000556618194668>):001:0> topic.last_read.class
=> Date
```

Refer rsim/oracle-enhanced#845
rails/rails#25897

Oracle 12.2+ supports 128 byte identifier length

* activesupport(class_attribute): Use redefine_singleton_method

* activesupport(class_attribute): Use keyword arguments

* activesupport(class_attribute): Avoid unnecessary redefinition for default

```ruby
$ ARCONN=oracle bin/test test/cases/migration/columns_test.rb -n test_rename_column_with_multi_column_index
... snip ...

F

Failure:
ActiveRecord::Migration::ColumnsTest#test_rename_column_with_multi_column_index [/home/yahonda/git/rails/activerecord/test/cases/migration/columns_test.rb:113]:
--- expected
+++ actual
@@ -1 +1 @@
-["i_test_models_hat_style_size"]
+["index_test_models_on_hat_style_and_size"]

```

Kind of reverting #9395
Refer https://github.com/rsim/oracle-enhanced/pull/1703

Add test and change how format set in ActionMailer

Previously this used self.formats= to set the format which render would
use to find templates. This worked, but was untested, and looked a
little confusing because it was doing the mutation within a loop.

This commit replaces the assignment with passing formats: [format] into
the render call, which makes it more obvious that that's the purpose of
the format. It also adds a test to verify the formats being used.

Allow nil format on templates

This was using a model without a default scope what made the test don't
break anymore if the code is removed.

Relax table name detection in `from` to allow any extension like INDEX hint

Enable SQL statement cache for `find` on base class as with `find_by`

#35360 allows table name qualified if `from` has original table name.
But that is still too strict. We have a valid use case that `from` with
INDEX hint (e.g. `from("comments USE INDEX (PRIMARY)")`).
So I've relaxed the table name detection in `from` to allow any
extension like INDEX hint.

Fixes #35359.

Add reselect method

`require "bcrypt"` is unsed since #15431.
`require "models/topic"` is unused since 893c647d.

Related and follows d333d852.

This is an alternative of 65c4b1b5.

https://buildkite.com/rails/rails/builds/59147#117e9445-23e8-455a-b486-ea0ae9636405/120-129