When a enum element contains the prefix 'not_'. I warns to users
to be aware of this new feature.

Example code:
    class Foo < ActiveRecord::Base
      enum status: [:sent, :not_sent]
    end

This makes to be able to ignore the query in `assert_queries` even if
accidentally reconnected a connection.

https://buildkite.com/rails/rails/builds/61917#4c49187a-3173-4d5c-8a8d-d65768f5bfc9/1000-1799

Introduced in bba7c63a

Before:

```
$ TESTOPTS="-n=/test_yaml_dump_and_load/" bundle exec rake
test:postgresql

:scisors: ... :scisors:

Using postgresql
Run options: -n=/test_yaml_dump_and_load/ --seed 36896

/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
/home/u/code/rails/activerecord/lib/active_record/connection_adapters/postgresql/column.rb:15:
warning: instance variable @serial not initialized
.

Finished in 0.195325s, 5.1197 runs/s, 35.8376 assertions/s.
1 runs, 7 assertions, 0 failures, 0 errors, 0 skips

```
Co-authored-by: NRyuta Kamizono <kamipo@gmail.com>

Fix broken url configs

This PR is to fix #36559 but I also found other issues that haven't been
reported.

The check for `(config.size == 1 && config.values.all? { |v| v.is_a?
String })` was naive. The only reason this passed was because we had
tests that had single hash size configs, but that doesn't mean we don't
want to create a hash config in other cases. So this now checks for
`config["database"] || config["adapter"] || ENV["DATABASE_URL"]`. In the
end for url configs we still get a UrlConfig but we need to pass through
the HashConfig to create the right kind of UrlConfig. The UrlConfig's
are really complex and I don't necessarily understand everything that's
needed in order to act the same as Rails 5.2.

I edited the connection handler test to demonstrate how the previous
implementation was broken when checking config size. Now old and new
tests pass so I think this is closer to 5.2.

Fixes #36559

Address to "DEPRECATION WARNING: Uniqueness validator will no longer enforce case sensitive comparison in Rails 6.1"

Caused by #36210.

Warn if we can't read the yaml to create database tasks

For multiple databases we attempt to generate the tasks by reading the
database.yml before the Rails application is booted. This means that we
need to strip out ERB since it could be reading Rails configs.

In some cases like https://github.com/rails/rails/issues/36540 the ERB
is too complex and we can't overwrite with the DummyCompilier we used in
https://github.com/rails/rails/pull/35497. For the complex causes we
simply issue a warning that says we couldn't infer the database tasks
from the database.yml.

While working on this I decided to update the code to only load the
database.yml once initially so that we avoid having to issue the same
warning multiple times. Note that this had no performance impact in my
testing and is merely for not having to save the error off somewhere.
Also this feels cleaner.

Note that this will not break running tasks that exist, it will just
mean that tasks for multi-db like `db:create:other_db` will not be
generated. If the database.yml is actually unreadable it will blow up
during normal rake task calls.

Fixes #36540

https://github.com/vhf/free-programming-books seems to have moved to
https://github.com/EbookFoundation/free-programming-books

[ci skip] Refer actionmailbox and actiontext in the guide

Replace the www.robotstxt.org URL with https one [ci skip]

The robots.txt site is moved permanently to https URL.

Rely on Kernel require instead of self require

Merge pull request #36210 from vishaltelangre/raise-record-invalid-when-associations-fail-to-save-due-to-uniqueness-failure

Fix: ActiveRecord::RecordInvalid is not raised when an associated record fails to #save! due to uniqueness validation failure

With this code (exctracted from derailed_benchmarks):

```ruby

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "rails", "~> 6.0.0.rc1", require: false
end

FILES = [
  "rails/engine/configuration",
  "rails/source_annotation_extractor",
  "active_support/deprecation"
]

module Kernel
  alias :original_require :require

  def require(file)
    Kernel.require(file)
  end

  class << self
    alias :original_require :require
  end
end

Kernel.define_singleton_method(:require) do |file|
  original_require(file)
end

FILES.each do |file|
  puts "requiring file: #{file}"
  require file
end
```

It fails with Rails 6 and the change introduced by 32065
```
requiring file: rails/engine/configuration
requiring file: rails/source_annotation_extractor
Traceback (most recent call last):
	11: from repro_derailed.rb:33:in `<main>'
	10: from repro_derailed.rb:33:in `each'
	 9: from repro_derailed.rb:35:in `block in <main>'
	 8: from repro_derailed.rb:21:in `require'
	 7: from repro_derailed.rb:30:in `block in <main>'
	 6: from repro_derailed.rb:30:in `require'
	 5: from /Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/railties-6.0.0.rc1/lib/rails/source_annotation_extractor.rb:8:in `<top (required)>'
	 4: from /Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/activesupport-6.0.0.rc1/lib/active_support/deprecation/proxy_wrappers.rb:10:in `new'
	 3: from /Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/activesupport-6.0.0.rc1/lib/active_support/deprecation/proxy_wrappers.rb:10:in `new'
	 2: from /Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/activesupport-6.0.0.rc1/lib/active_support/deprecation/proxy_wrappers.rb:125:in `initialize'
	 1: from /Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/activesupport-6.0.0.rc1/lib/active_support/deprecation/proxy_wrappers.rb:23:in `method_missing'
/Users/benoit.tigeot/.rvm/gems/ruby-2.5.1/gems/activesupport-6.0.0.rc1/lib/active_support/deprecation/proxy_wrappers.rb:148:in `warn': private method `warn' called for nil:NilClass (NoMethodError)
```

Related:
- https://github.com/schneems/derailed_benchmarks/pull/130
- https://github.com/rails/rails/pull/32065

Also deduplicate schema cache data when using the init_with interface

Make `ActionDispatch::Response#content_type` behavior configurable

I changed return value of `ActionDispatch::Response#content_type` in #36034.
But this change seems to an obstacle to upgrading. https://github.com/rails/rails/pull/36034#issuecomment-498795893

Therefore, I restored the behavior of `ActionDispatch::Response#content_type`
to 5.2 and deprecated old behavior. Also, made it possible to control the
behavior with the config.

Address test_statement_cache_with_in_clause failure

Revert schema dumper to use strings rather than integers

This failure is occasional, does not always reproduce.

```ruby
$ cd activerecord
$ bundle exec rake test_postgresql
... snip ...

....F

Failure:
ActiveRecord::BindParameterTest#test_statement_cache_with_in_clause [/home/yahonda/git/rails/activerecord/test/cases/bind_parameter_test.rb:97]:
Expected: [1, 3]
  Actual: [3, 1]

rails test home/yahonda/git/rails/activerecord/test/cases/bind_parameter_test.rb:93

```

I think we should change this, but not in 6-0-stable since that's
already in RC and I was trying to only make changes that won't require
any app changes.

This reverts a portion of https://github.com/rails/rails/pull/36439 that
made all schema migration version numbers get dumped as an integer.
While it doesn't _really_ matter it did change behavior. We should bring
this back in 6.1 with a deprecation.

Add test cases to ensure deterministic order for ordinal methods

Include BCC in the mail that sent from the development page

The BCC should be included as we show input field for BCC in view.
https://github.com/rails/rails/blob/bf625f7fecabbcda22b388e088ad5c29016b2385/actionmailbox/app/views/rails/conductor/action_mailbox/inbound_emails/new.html.erb#L21-L24

Allow using env var to specify pidfile

Stop serializing and parsing columns_hash in Active Record schema caches

Autoload SyntaxErrorInTemplate

When a SyntaxError is detected in a template we raise this exception. On
a first request to the server the exception we get a NameError since the
exception is not required from `active_view/template/error.rb` yet.
However later on it gets required and a second request will succeed.
On the first request we see the rails "Something Wen Wrong" page and not
the expected syntax error in template error page with the webconsole and
stacktrace. By autoloading the constant we fix this issue.
Co-authored-by: NGannon McGibbon <gannon.mcgibbon@gmail.com>

Better error message for calling columns_hash

When a record does not have a table name, as in the case for a record
with `self.abstract_class = true` and no `self.table_name` set the error
message raises a cryptic:
"ActiveRecord::StatementInvalid: Could not find table ''" this patch now
raises a new `TableNotSpecified Error`

Fixes: #36274
Co-Authored-By: NEugene Kenny <elkenny@gmail.com>