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.

Dirty checking keeps a hash where the keys are the column name and the
value is a dup of the value from the database[1].  This hash is kept for
every AR object, which means that we dup every column name for every AR
object that does dirty checking.  Freezing the column name prevents the
column name from being duped and reduced overall string allocations.

Here is a benchmark to demonstrate:

```ruby
require 'active_record'

class Topic < ActiveRecord::Base
end

20.times do |i|
  Process.waitpid fork {
    ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

    ActiveRecord::Base.connection.instance_eval do
      create_table(:topics) do |t|
        t.string   :title, limit: 250
        t.string   :author_name
        t.string   :author_email_address
        t.string   :parent_title
        t.string   :type
        t.string   :group
        i.times do |j|
          t.string :"aaa#{j}"
        end
        t.timestamps null: true
      end
    end

    ObjectSpace::AllocationTracer.setup(%i{type})

    Topic.create title: "aaron" # heat cache

    result = ObjectSpace::AllocationTracer.trace do
      10.times do |i|
        Topic.create title: "aaron #{i}"
      end
    end

    puts "#{Topic.columns.length},#{(result.find { |k,v| k.first == :T_STRING }.last.first / 10)}"
  }
end
```

1. https://github.com/rails/rails/blob/3ad381c3f8598d9920998c8949a96b5f62b280dd/activerecord/lib/active_record/attribute_set/builder.rb#L102

migrations Class methods have nodoc, fix it for API [ci skip]

The focus of this change is to make the API more accessible.
References to method and classes should be linked to make it easy to
navigate around.

This patch makes exzessiv use of `rdoc-ref:` to provide more readable
docs. This makes it possible to document `ActiveRecord::Base#save` even
though the method is within a separate module
`ActiveRecord::Persistence`. The goal here is to bring the API closer to
the actual code that you would write.

This commit only deals with Active Record. The other gems will be
updated accordingly but in different commits. The pass through Active
Record is not completely finished yet. A follow up commit will change
the spots I haven't yet had the time to update.

/cc @fxn

Documentation ActiveRecord Attributes API code fix

add application_job.rb to template of mountable engine

Fix formatting of ActiveRecord PostgreSQL guide.

Add basic support for access control headers to ActionDispatch::Static

Only prepend a single module when defining deprecation wrappers.

I could not find any reason why each method got its own prepended
module here, and all tests appear to pass with my change.

The rdoc parser seems to trip on the `private def` construct.
Public methods following a method defined with `private def` are not
visible inside the module docs but are appended to the top-most module.

For example the method `ActiveRecord::QueryMethods#distinct` was listed
under `ActiveRecord#distinct`.

/cc @sgrif

[ci skip]

This class is only used internally. We should keep it out of public
documentation. This patch adds nodoc for
`ActiveRecord::Associations::Builder` and everything nested within.

This error is raised in certain situations when eager loading
polymorphic associations. We even mention it in our docs. It should be
included in our API.

Conflicts:
	activerecord/lib/active_record/associations.rb

Also unify the format of code example output. Only use `# =>` if the
actual return value is described. Otherwise simply use `#`.

Conflicts:
	activerecord/lib/active_record/relation/query_methods.rb

Closes #21563.

The `name` argument of `add_references` was both used to generate the
column name `<name>_id` and as the target table for the foreign key
`name.pluralize`.

It's primary purpose is to define the column name. In cases where the
`to_table` of the foreign key is different than the column name we
should be able to specify it individually.

Move the methods for schema dumping into `{mysql,postgresql}/schema_dumper.rb`

Allow mysql and mysql2 tests run by database user with password

ActionMailer https on URL with force_ssl = true

Current master branch includes many schema dumping improvements.
It extract these features to the appropriate files.

Remove deprecated pg_dump -i flag

Move schema definiton classes into the appropriate files

Change `Journey::Route#verb` to return string instead of regexp.

* Revises the name of Rails components (they have a space).

* Uniform word wrap at column 80..

* Uniform punctuation, according to our guidelines.

* Minor edits of details seen in passing.

do not generate manifest.js in plugins

[ci skip] Fix typo in deep_dup docs

Since the plugin generator do not generate assets, I think manifest.js also that it unnecessary.