The documentation previously claimed that `db/schema.rb` was "the
authoritative source for your database schema" while simultaneously
also acknowledging that the file is generated. These two statements are
incongruous and the guides accurately call out that many database
constructs are unsupported by `schema.rb`. This change updates the
comment at the top of `schema.rb` to remove the assertion that the file
is authoritative.

The documentation also previously referred vaguely to "issues" when
re-running old migrations. This has been updated slightly to hint at the
types of problems that one can encounter with old migrations.

In sum, this change attempts to more accurately capture the pros, cons,
and shortcomings of the two schema formats in the guides and in the
comment at the top of `schema.rb`.

[Derek Prior & Sean Griffin]
Co-authored-by: NSean Griffin <sean@seantheprogrammer.com>

To solve the problem #32299, just enough to introduce
`fk_ignore_pattern` option.

I don't think there is a need to expose these constants.

This makes more sense, as the foreign key ignore pattern is only used by
the schema dumper.

When dumping the database schema, Rails will dump foreign key names only
if those names were not generate by Rails. Currently this is determined
by checking if the foreign key name is `fk_rails_` followed by
a 10-character hash.

At [Cookpad](https://github.com/cookpad), we use
[Departure](https://github.com/departurerb/departure) (Percona's
pt-online-schema-change runner for ActiveRecord migrations) to run migrations.
Often, `pt-osc` will make a copy of a table in order to run a long migration
without blocking it. In this copy process, foreign keys are copied too,
but [their name is prefixed with an underscore to prevent name collision
](https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html#cmdoption-pt-online-schema-change-alter-foreign-keys-method).

In the process described above, we often end up with a development
database that contains foreign keys which name starts with `_fk_rails_`.
That name does not match the ignore pattern, so next time Rails dumps
the database schema (eg. when running `rake db:migrate`), our
`db/schema.rb` file ends up containing those unwanted foreign key names.
This also produces an unwanted git diff that we'd prefer not to commit.

In this PR, I'd like to suggest a way to expose the foreign key name
ignore pattern to the Rails configuration, so that individual projects
can decide on a different pattern of foreign keys that will not get
their names dumped in `schema.rb`.

Rails has some support for multiple databases but it can be hard to
handle migrations with those. The easiest way to implement multiple
databases is to contain migrations into their own folder ("db/migrate"
for the primary db and "db/seconddb_migrate" for the second db). Without
this you would need to write code that allowed you to switch connections
in migrations. I can tell you from experience that is not a fun way to
implement multiple databases.

This refactoring is a pre-requisite for implementing other features
related to parallel testing and improved handling for multiple
databases.

The refactoring here moves the class methods from the `Migrator` class
into it's own new class `MigrationContext`. The goal was to move the
`migrations_paths` method off of the `Migrator` class and onto the
connection. This allows users to do the following in their
`database.yml`:

```
development:
  adapter: mysql2
  username: root
  password:

development_seconddb:
  adapter: mysql2
  username: root
  password:
  migrations_paths: "db/second_db_migrate"
```

Migrations for the `seconddb` can now be store in the
`db/second_db_migrate` directory. Migrations for the primary database
are stored in `db/migrate`".

The refactoring here drastically reduces the internal API for migrations
since we don't need to pass `migrations_paths` around to every single
method. Additionally this change does not require any Rails applications
to make changes unless they want to use the new public API. All of the
class methods from the `Migrator` class were `nodoc`'d except for the
`migrations_paths` and `migrations_path` getter/setters respectively.

Add support for specifying non-default operator classes in PostgreSQL
indexes. An example CREATE INDEX query that becomes possible is:

    CREATE INDEX users_name ON users USING gist (name gist_trgm_ops);

Previously it was possible to specify the `gist` index but not the
custom operator class. The `add_index` call for the above query is:

    add_index :users, :name, using: :gist, opclasses: {name: :gist_trgm_ops}

and abstract `SchemaDumper#extensions` is now an empty method.

Since #30337, every database adapter has its own `SchemaDumper`.
`extensions` are only supported by PostgreSQL database and postgresql database adapter.

I was added a table options after `force: :cascade` in #17569 for not
touching existing tests (reducing diff). But `force: :cascade` is not an
important information. So I prefer to place a table options before
`force: :cascade`.

Currently `SchemaDumper` is only customizable for column options. But
3rd party connection adapters (oracle-enhanced etc) need to customizable
for table or index dumping also. To make it possible, I introduced
adapter specific `SchemaDumper` classes for that.

MySQL 5.7 and PostgreSQL 9.6 allow table identifiers have the dollar sign.

* MySQL 5.7
https://dev.mysql.com/doc/refman/5.7/en/identifiers.html

> Permitted characters in unquoted identifiers:
>  ASCII: [0-9,a-z,A-Z$_] (basic Latin letters, digits 0-9, dollar, underscore)

* PostgreSQL 9.6
https://www.postgresql.org/docs/9.6/static/sql-syntax-lexical.html

> SQL identifiers and key words must begin with a letter (a-z, but also letters with diacritical marks and non-Latin letters) or an underscore (_). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), or dollar signs ($). Note that dollar signs are not allowed in identifiers according to the letter of the SQL standard, so their use might render applications less portable. The SQL standard will not define a key word that contains digits or starts or ends with an underscore, so identifiers of this form are safe against possible conflict with future extensions of the standard.

Address #30044

[Yasuo Honda & Ryuta Kamizono]

The list of enabled adapter extensions in the schema dump isn't
sorted by default, so it may happen that the sorting changes
over time. If you're using a VCS, a change to the sorting results
in a diff without any real change. Sorting the list should solve
this problem.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

* Fix the comment on `formatted_version`
* Extract `define_params`
* Remove duplicated guard clause for `@version`

`tables` returns only tables now.

Add `default_index_type?` to the generic schema dumper doesn't have the knowledge about an index type

Follow up to #26735.

If `table_options` returns `{ comment: nil }`, `create_table` line is
broken.

Example:

```ruby
  create_table "accounts", force: :cascade,  do |t|
```

`migration_keys` includes `name` but `name` is not a column option.

```ruby
  # Before
  t.index ["firm_id", "type", "rating"], name: "company_index", order: {"rating"=>:desc}, using: :btree

  # After
  t.index ["firm_id", "type", "rating"], name: "company_index", order: { rating: :desc }, using: :btree
```

Currently blank comment does not dump to `db/schema.rb`. But created it
even if specified blank.

`format_string` is used for standardized column types/arguments spaces.
Now the standardization was removed at df84e986.

Now the schema dumper by default doesn't align the types and arguments
in the ruby format anymore.

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.

Actually schema dumper/creation supports composite primary key (#21614).
Therefore it should not show the warning about composite primary key in
connection adapter.

This change moves the warning to `AttributeMethods::PrimaryKey` and
suppress the warning for habtm join table.

Fixes #25388.

Rails 5.0 has been dropped Ruby 1.9 support.
I think no need magic comment anymore.

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

* Switch to keyword args where we can without breaking compat.
* Use add_table_options! for :options, too.
* Some code polish.