Follow up to 99cf7558.

- These tests were fixed earlier on master in https://github.com/rails/rails/commit/f13ec72664fd13d33d617103ca964a7592295854.
- They started failing in first place due to change in https://github.com/brianmario/mysql2/commit/f14023fcfee9e85e6fc1b0e568048811518f8c23.
- They will fail again when the message is changed in mysql2 so let's
  not rely on the error message.

Test: JSON attribute value nil can be used in where(attr: nil)

Add changelog entry

All indentation was normalized by rubocop auto-correct at 80e66cc4.
But comments was still kept absolute position. This commit aligns
comments with method definitions for consistency.

By doing `@connection = nil` that means that we need nil checks before it
is used anywhere, but we weren't doing those checks.  Instead, we get a
NoMethodError after using a connection after it fails to reconnect.

Neither of the other adapters set @connection to nil, just the mysql2
adapter. By just closing it, we avoid the need to check if we have a
connection object and it will produce an appropriate exception when used.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Fixes #25300.

Follow up of #20815.

```ruby
class CreatePeople < ActiveRecord::Migration[5.0]
  def change
    create_table :people do |t|
      t.integer :int
      t.bigint :bint
      t.text :txt
      t.binary :bin
    end
  end
end
```

Result.

In postgresql and sqlite3 adapters:

```ruby
ActiveRecord::Schema.define(version: 20160531141018) do

  create_table "people", force: :cascade do |t|
    t.integer "int"
    t.bigint  "bint"
    t.text    "txt"
    t.binary  "bin"
  end

end
```

In mysql2 adapter:

```ruby
ActiveRecord::Schema.define(version: 20160531141018) do

  create_table "people", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4" do |t|
    t.integer "int"
    t.bigint  "bint"
    t.text    "txt",  limit: 65535
    t.binary  "bin",  limit: 65535
  end

end
```

After this patch:

```ruby
ActiveRecord::Schema.define(version: 20160531141018) do

  create_table "people", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4" do |t|
    t.integer "int"
    t.bigint  "bint"
    t.text    "txt"
    t.binary  "bin"
  end

end
```

Fix failing tests

Refactor of #22911.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

Others adapters need to perform a check for validity.
Add coverage for mysql2 db type validation

Previously we were assuming that the only valid types for encoding were
arrays and hashes. However, any JSON primitive is an accepted value by
both PG and MySQL.

This does involve a minor breaking change in the handling of `default`
in the schema dumper. This is easily worked around, as passing a
hash/array literal would have worked fine in previous versions of Rails.
However, because of this, I will not be backporting this to 4.2 or
earlier.

Fixes #24234

We support microsecond datetime precision for MySQL 5.6.4+. MariaDB has
supported it since 5.3.0, but even 10.x versions return a compatible
version string like `5.5.5-10.1.8-MariaDB-log` which we parse as 5.5.5,
before MySQL supported microsecond precision.

Specialize our version check to account for MariaDB to fix.

Because we define `QUOTED_TRUE` as `"1"` and `QUOTED_FALSE` as `"0"`.
And add test cases to ensure this commit does not break current
behavior even if the value of `attributes_before_type_cast` is false.

For keep the default SQL mode.

Before:

```ruby
create_table "big_numbers", force: :cascade do |t|
  t.integer "bigint_column", limit: 8
end
```

After:

```ruby
create_table "big_numbers", force: :cascade do |t|
  t.bigint "bigint_column"
end
```

Currently `exec_query` raises `NoMethodError` when executing no result
queries (`INSERT`, `UPDATE`, `DELETE`, and all DDL) in mysql2 adapter.

```
irb(main):002:0> conn.execute("create table t(a int)")
   (43.3ms)  create table t(a int)
=> nil
irb(main):003:0> conn.execute("insert into t values (1)")
   (19.3ms)  insert into t values (1)
=> nil
irb(main):004:0> conn.exec_query("insert into t values (1)")
  SQL (28.6ms)  insert into t values (1)
NoMethodError: undefined method `fields' for nil:NilClass
```

Follow up to #13040.

`initialize_schema_migrations_table` is called in every migrations.

https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/migration.rb#L1080
https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/schema.rb#L51

This means that extra `show variables` is called regardless of the
existence of `schema_migrations` table.

This change is to avoid extra `show variables` if `schema_migrations`
table exists.

Follow up to #22896.

Mysql has a weird bug where it cannot index a string column of utf8mb4 if it is over a certain character limit. To get compatibility with msql we can add a limit to the key column. 191 characters is a very long key, it seems reasonable to limit across all adapters since using a longer key wouldn't be supported in mysql.

Thanks to @kamipo for the original PR and the test refactoring.

Conversation: https://github.com/rails/rails/pull/23009#issuecomment-171416629

Avoid instanciate `ActiveRecord::Result` and calling
`ActiveRecord::Result#hash_rows` for the performance.

In MySQL 5.7.5 and up, ONLY_FULL_GROUP_BY affects handling of queries
that use DISTINCT and ORDER BY. It requires the ORDER BY columns in the
select list for distinct queries, and requires that the ORDER BY include
the distinct column.

See https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

Follow up to #22642.