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>

Removes default ENGINE documentation hint for MySQL and MariaDB as we
aren't taking measures to set the default ENGINE anymore.

Sentence had some awkward grammar and was kind of confusing, tried to make it easier
to understand what it was trying to say :)

[ci skip]

[ci skip]

This was pointed out on
https://github.com/rails/rails/issues/15583#issuecomment-239212831

The comment was lost in the changes made by
fcd0ac06 and
3e1373a7

In #23179 the migration generator was changed to no longer output `index: true` for `references` migrations. This updates the migrations guide to remove `index: true` from relevant examples.

[ci skip]

- Add link for finding the addional options for index.
- Add example for unique index as this is a common requirement.
- Add link in guide for index options.

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

- We no longer add `null: false` for timestamps columns as per
  rails/rails@a939506.
- Followup of
  https://github.com/rails/docrails/commit/14867b1a9af813b0147bba2ae06675137d61d77b.

Fixed errors in rails migrations docs [ci skip]

Fixed errors in rails security docs [ci skip]

I go through the `http://edgeguides.rubyonrails.org/` and found `rake` commands in various files that are in RAILS 5.0 implement by `bin/rails` command. I try to change all that can be directly use `bin/rails …`

Still more to do. Please assist!

Even though this means more things to change when we bump after a
release, it's more important that our examples are directly copyable.

If we use a real version, at best that'll be an onerous update required
for each release; at worst, it will encourage users to write new
migrations against an older version than they're using.

The other option would be to leave these bare, without any version
specifier. But as that's just a variant spelling of "4.2", it would seem
to raise the same concerns as above.

Add 'db/'; corrects structure.rb to structure.sql

I added that *structure.sql* file can be used when *db:reset* is run.

*db:reset* tasks states *db:reset* loads database from *db/schema.rb*
or *db/structure.sql* depending on the configuration (although
*db/schema.rb* is the default), hence the change.

* Documentations and comments about what methods
  `Migration#change` can reverse is out of date.
  For example `change_column_default` is now reversible
  by this [commit](https://github.com/rails/rails/pull/20018).

* Comments about `CommandRecorder` dose not match with Rails Guide.
  For example `add_foreign_key` is listed only on Rails Guide.

In rails generally migration file's timestamp is "YYYYMMDDHHMMSS".

this behavior was changed in b8e1f202

* insert 'and' into a poorly formed sentence.
* avoid using 'reverse' twice in the same sentence (which would indicate
  the opposite of what is meant)

- Changed `IN` to `ON` in all note sentences in guides.