[ci skip] Fix typos

That is happened with `distinct`, and that is not only happened on MyS
L, but also on PostgreSQL, SQL Server, and Oracle.

[ci skip]

[ci skip]Fix typo:  constaint -> constraint

Fix including/excluding flattening

Update links and code examples in the guides to use HTTPS

Load YAML for rake tasks without parsing ERB

Although the old name had a certain persistence, this ain't the kind of
file we're in now.

Foreign keys could be created to the same table.
So `remove_foreign_key :from_table, :to_table` is sometimes ambiguous.
This allows `remove_foreign_key` to remove the select one on the same
table with giving both `to_table` and `options`.

This change adds a new method that loads the YAML for the database
config without parsing the ERB. This may seem odd but bear with me:

When we added the ability to have rake tasks for multiple databases we
started looping through the configurations to collect the namespaces so
we could do `rake db:create:my_second_db`. See #32274.

This caused a problem where if you had `Rails.config.max_threads` set in
your database.yml it will blow up because the environment that defines
`max_threads` isn't loaded during `rake -T`. See #35468.

We tried to fix this by adding the ability to just load the YAML and
ignore ERB all together but that caused a bug in GitHub's YAML loading
where if you used multi-line ERB the YAML was invalid. That led us to
reverting some changes in #33748.

After trying to resolve this a bunch of ways `@tenderlove` came up with
replacing the ERB values so that we don't need to load the environment
but we also can load the YAML.

This change adds a DummyCompiler for ERB that will replace all the
values so we can load the database yaml and create the rake tasks.
Nothing else uses this method so it's "safe".

DO NOT use this method in your application.

Fixes #35468

[ci skip] Fix typo `beacuse` -> `because`

To avoid newly adding the warnings, which are frequently addressed.

ac721c85
951383bd
8a0f235f

c33c03e8
424b2019

https://github.com/rails/rails/pull/35451#discussion_r262746834

Since #23461, all adapters supports prepared statements, so that clears
the prepared statements cache is no longer database specific.

Actually, I struggled to identify the cause of random CI failure in
#23461, that was missing `@statements.clear` in `clear_cache!`.

This extracts `clear_cache!` to ensure the common concerns in the
abstract adapter.

Adds a method to ActiveRecord allowing records to be inserted in bulk without instantiating ActiveRecord models. This method supports options for handling uniqueness violations by skipping duplicate records or overwriting them in an UPSERT operation.

ActiveRecord already supports bulk-update and bulk-destroy actions that execute SQL UPDATE and DELETE commands directly. It also supports bulk-read actions through `pluck`. It makes sense for it also to support bulk-creation.

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).