Sorry for forgetting to include it in my original PR 😬

r? @rafaelfranca

[ci skip]

Follow up to 98264a13

Example:

    create_table :users do |t|
      t.string :name
      t.index 'lower(name) varchar_pattern_ops'
    end

Fixes #19090.
Fixes #21765.
Fixes #21819.
Fixes #24359.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

If the adapter supports indexes in create table, it generates SQL
in one query.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

[ci skip]

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.

The option backed to `start` in da269343.

Forgot to add, sigh.

Closes #24273

When a proc is given as a default value, the form builder ends up
displaying `Proc#to_s` when the default is used. That's because we
didn't handle the proc until type casting. This issue technically can
occur any time that a proc is the value before type casting, but in
reality the only place that will occur is when a proc default is
provided through the attributes API, so the best place to handle this
edge case is there.

I've opted to memoize instead of just moving the `Proc#call` up, as this
made me realize that it could potentially interact very poorly with
dirty checking.

The code here is a little redundant, but I don't want to rely on how
`value_before_type_cast` is implemented in the super class, even if it's
just an `attr_reader`.

Fixes #24249

Close #24306

[ci skip]

Sync AV, AR, AJ, AS, AM changelogs with our 5.0 release notes draft.
This is a follow up to c94045dc and contains changes made since the
release of beta1.

related to #19324 and #18606

[skip ci]

Closes #23934.

This is a forward port of ac832a43

Previously the scope of all associations with extensions were wrapped in
an instance dependent proc. This made it impossible to preload such
associations.

Originally, `{insert|update|delete}_sql` is protected methods.
We can use the `{insert|update|delete}` public methods instead.

I ran into an issue where validations on a suppressed record were
causing validation errors to be thrown on a record that was never going
to be saved.

There isn't a reason to run the validations on a record that doesn't
matter.

This change moves the suppressor up the chain to be run on the `save` or
`save!` in the validations rather than in persistence. The issue with
running it when we hit persistence is that the validations are run
first, then we hit persistance, and then we hit the suppressor. The
suppressor comes first.

The change to the test was required since I added the
`validates_presence_of` validations. Adding this alone was enough to
demonstrate the issue. I added a new test to demonstrate the new
behavior is explict.

Adds changelog headers for beta3 release

I had to revert changes made for this CHANGELOG entry so this is no
longer valid. The change for this entry was removed in 2c02bc0a.

When `ActiveRecord::Coders::JSON` serialization is used and the default of the column returns `''` it raises the following error:

```
JSON::ParserError: A JSON text must at least contain two octets!
```

If MySQL is running in non-strict mode, it returns an empty string as column default for a text column:

```ruby
def extract_default
  if blob_or_text_column?
    @Default = null || strict ? nil : ''
  end
end
```

Since `''` is invalid JSON, there shouldn't be an attempt to parse it, it should be treated like nil.
ActiveRecord::Coders::JSON should behave consistently for all possible non-user-set column default values.

`#` was removed at f57092ad but this `#` is intentional.
e.g. https://github.com/rails/rails/blame/v5.0.0.beta2/activerecord/CHANGELOG.md#L1423-L1426

This was showing up as an H1. Fixing the indentation here fixes the codeblock.

When passing in an array of different types of objects to `where`, it would only take into account the class of the first object in the array.

    PriceEstimate.where(estimate_of: [Treasure.find(1), Car.find(2)])
	# => SELECT "price_estimates".* FROM "price_estimates"
         WHERE ("price_estimates"."estimate_of_type" = 'Treasure' AND "price_estimates"."estimate_of_id" IN (1, 2))

This is fixed to properly look for any records matching both type and id:

    PriceEstimate.where(estimate_of: [Treasure.find(1), Car.find(2)])
    # => SELECT "price_estimates".* FROM "price_estimates"
         WHERE (("price_estimates"."estimate_of_type" = 'Treasure' AND "price_estimates"."estimate_of_id" = 1)
         OR ("price_estimates"."estimate_of_type" = 'Car' AND "price_estimates"."estimate_of_id" = 2))

[ci skip]

Previously if you used `t.foreign_key` twice within the same
`create_table` block using the same `to_table`, all statements except
the final one would fail silently. For example, the following code:

    def change
      create_table :flights do |t|
        t.integer :from_id, index: true, null: false
        t.integer :to_id,   index: true, null: false

        t.foreign_key :airports, column: :from_id
        t.foreign_key :airports, column: :to_id
      end
    end

Would only create one foreign key, on the column `from_id`.

This commit allows multiple foreign keys to the same table to be created
within one `create_table` block.

The author name was lost in the merge commit 6fedc7d5.