Reformatted two SQL queries in Section 6) and 7) to avoid horizontal
scrolling where possible.

response in integration test accessible without @ via attr_reader

Revert "Added documentation to explain what happens if you don't set any values for the dependent option on has_many"

This reverts commit 820a677d.

Added documentation to explain what happens if you don't set any values for the dependent option on has_many

parameter passed to the ActiveSupport::Notifications.subscribe
method instead of a block

Example code sample and output is provided as well.

Improve the performance of #delegate

Some refactor for association.

* Remove unused association_class method.
* Remove a unnecessary assignment.
* Move @updated to BelongsToAssociation that only reference this instance variable.
* Reset @stale_state at the reset method. I think this place is right place.

Remove the use of #__send__ in order to boost performance. This also
means that you can no longer delegate to private methods on the
target object.

Add method FormBuilder#index

Add index method to FormBuilder. Useful when you use field_for and need to know the index number into the iteration.

Add better errors reporting for ActiveModel::Configuration

Get a properly aliased_table_name, when we use a polymorphic association.

Override AR::Relation methods in NullRelation.

Could also be worded 'as an option', but plural made more sense given 
the mention of ':handlers and :formats'.

This reverts commit f28f1d70.

Reason: This is already covered in this guide elsewhere.

no need in temporary array in number_to_phone helper

So a NullRelation (Relation#none) is chainable with database methods.

Corrected grammatical errors in schema_dumper and scoping/default

Fix delete_all when chained with joins.

Closes #5202 and #919