Conflicts:
	guides/source/testing.md

this allows us to avoid nil checks on the return value

Ruby generally does not use the is_* prefix on predicate methods.

Null transaction

Transactions refactoring - 2

there's no reason to to_sym the string if it doesn't match the regexp
anyway

hash lookup should be faster than searching an array.

Add ActiveRecord::Tasks::DatabaseTasks.migrate

[doc] Clarify how the ConnectionHandler works

This makes the implicit description of how connection pooling works a
little more explicit. It converts the examples of a model hierarchy into
actual Ruby code and demonstrates how the key structure of the
database.yml relates to the `establish_connection` method.

This extracts the logic that was embedded in a Rake task into a static
method.
Bonus: the first test for `rake db:migrate`

Refactor join_keys to remove complex conditionals

These methods mutate the path variable/argument so there is no need
to reassign it every time.

Pushing conditionals down to through reflection

Only the through association needs the part of this conditional
that deals with belongs to and polymorphic? so that can be pushed
down into the ThroughReflection reducing the conditionals.

Remove conditional because we can delegate join keys to source reflection

Remove need for source_macro checking

By adding join_id_for to the other reflections we remove the need
to cehck against the source_macro and can reduce the conditioanl
from the original join_id_for(owner)

Using polymorphism instead of testing the source_macro

This case statement in join_association is almost exactly the same
as the original join_keys code. Testing taht theory by creating a
new join_dependency_keys(assoc_klass) method.

Refactor join_keys further to be more concise

Fixed format of "#:nodoc:" to "# :nodoc:" where I added them to this
file.

Merge pull request #16351 from eileencodes/finish-refactoring-macro-to-eliminate-checking-against-symbols

Redefine macro checks for reflections

This is not storying the RouteSet instance anywhere as the other
examples in the file, so no need to use #tap.

Add tests to ensure default proc is used when `HashWithIndifferentAccess' is initialized with a block

Add tests to ensure default proc is used when `HashWithIndifferentAccess' is initialized with a block