When a has_many association is destroyed by `dependent: destroy`,
destroyed_by_association is set to the reflection, and this can be
checked in callbacks.  This matches that behaviour for has_one
associations.

Add purpose and expiry to messages encrypted using Message Encryptor

Fix `warning: circular argument reference`

Create `rails@localhost` user on travis ci

[ci skip]Replace jquery-ujs with rails-ujs

Looks like that latest CI failure is due to missing `rails@localhost`.
Creating the user may solve the issue.

https://travis-ci.org/rails/rails/jobs/254996063#L831

This fixes the following warnings:

```
rails/activejob/lib/active_job/test_helper.rb:119: warning: circular argument reference - except
rails/activejob/lib/active_job/test_helper.rb:166: warning: circular argument reference - except
```

Make preload query to preparable

Fix type casting a time for MariaDB

Context #24542.

Since 8ebe1f2f, it has lost stripping date part for a time value. But I
confirmed it is still needed even if MariaDB 10.2.6 GA.

MariaDB 10.2.6, `prepared_statements: true`:

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/time_precision_test.rb -n test_formatting_time_according_to_precision
Using mysql2
Run options: -n test_formatting_time_according_to_precision --seed 37614

F

Failure:
TimePrecisionTest#test_formatting_time_according_to_precision [test/cases/time_precision_test.rb:53]:
Failed assertion, no message given.

bin/rails test test/cases/time_precision_test.rb:46

Finished in 0.040279s, 24.8268 runs/s, 24.8268 assertions/s.

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
```

Assign `content_type` only once

Bugfix: unscope(where: [columns]) would not remove the correct binds

Catch postgres connection errors when trying to dealloc

Add `except` option for ActiveJob::TestHelper methods

Fix unscoping `default_scope` in STI associations

Dates are able to be natively handled by the mysql2 gem. libmysql (and
the wire protocol) represent each portion of the date as an integer,
which is significantly faster to encode and decode. By passing the Ruby
date objects through directly, we can save a good bit of time and
memory.

Since 5c71000d, it has lost to be able to unscope `default_scope` in STI
associations. This change will use `.empty_scope?` instead of
`.values.empty?` to regard as an empty scope if only have
`type_condition`.

Fix typo s/rause/raise/

This method needs to be protected.

Fix `JoinDependency` with using a custom table

[ci skip] CodeTriage badge

[Code Triage](https://www.codetriage.com/) is an app I've maintained for the past 4-5 years with the intent of getting people involved in open source. It sends subscribers a random open issue for them to help "triage". For Ruby projects such as "rails/rails" you can also subscribe to documentation. For example you can get a few random documented methods, or if you want to write docs, get undocumented methods.

The initial approach was inspired by seeing the work of the small core team spending countless hours asking "what rails version was this in" and "can you give us an example app". The idea is to outsource these small interactions to a huge team of volunteers and let the core team focus on their work.

The purpose of the badge is to give more people an easier way to start contributing to Rails. Here's what it currently looks like:

[![Code Triage Badge](https://www.codetriage.com/rails/rails/badges/users.svg)](https://www.codetriage.com/rails/rails)

The number is how many people are currently subscribed (a.k.a. "helpers") to the project on CodeTriage, the color is based off of the number of open issues in the project. You can see an example of this badge on another popular open source repo [Crystal](github.com/crystal-lang/crystal/).

> For context to non-rails core: I also maintain sprockets (though a release hasn't happened in some time, sorry), and I have commit to Rails. I'm not some rando trying to push arbitrary links to READMEs on GitHub.

support `-` as an argument to `rails runner`

in Rails 4.0, you could use `/dev/stdin` on both Linux and Mac, but with
the switch to Kernel.load in Rails 4.1, this broke on Linux (you get
a LoadError). Instead, explicitly detect `-` as meaning stdin, then
read from stdin explicitly, instead of performing file gymnastics. This
should now work on any platform uniformly.

Passing a script via stdin is useful when you're sshing to a server,
and the script you want to run is stored locally. You could theoretically
pass the entire script on the command line, but in reality you'll run
into problems with the command being too long.

Clarifies the intent that aren't just loading the
model but really caring about triggering the on_load
callbacks.

Without this fix, `JoinDependency` doesn't use a custom table alias:

```
% ARCONN=sqlite3 be ruby -w -Itest test/cases/relations_test.rb -n test_using_a_custom_table_with_joins_affects_the_wheres
Using sqlite3
Run options: -n test_using_a_custom_table_with_joins_affects_the_wheres --seed 14531

E

Error:RelationTest#test_using_a_custom_table_with_joins_affects_the_wheres:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: posts.author_id: SELECT  "omg_posts".* FROM "posts" "omg_posts" INNER JOIN "authors" ON "authors"."id" = "posts"."author_id" WHERE "omg_posts"."title" = ? LIMIT ?
```

Extract `NullRelationTest` from `RelationTest`

Remove outdated `test_scoped_responds_to_delegated_methods`

Correctly handle frozen options for ActiveRecord::Serialization#seria…

Fix HashWithIndifferentAccess#default when include?(nil)

Fix `where` with a custom table

`Persistence#delete` should not be affected by scoping