additional note about routes in functional tests for engines [ci skip]
Improve Contributing to Rails Guide

Logically group the content, so it makes more sense if someone tries to read from start of page to end of page. [ci skip]
Add documentation for Thread#freeze
Instead boat load of libraries, simply say StdLib
Fix error for aggregate methods with select, see issue #13648
fix exception on polymorphic associations with predicates
Merge pull request #14711 from swoker/activerecord_fix_aggregate_methods_with_select

Activerecord fix aggregate methods with select
:scissors:
New CHANGELOG entries are in to top of file
Use assert_not
Merge branch 'rm-fix-13648'

Includes https://github.com/rails/rails/pull/14711 and some cleanup
commits.

Fixes #13648
Use path where extensions are defined, not used

For consistency with all other documentation, point to where these core
extensions are defined, not where the example code comes from.

[ci skip]
Document LoadError core extensions

[ci skip]
Fix a few grammar issues. [ci skip]
Correct docs as singlton true is not returned here

[ci skip]
The comparison between `Relation` and `CollectionProxy` should be consistent.

Example:

    author.posts == Post.where(author_id: author.id)
    # => true
      Post.where(author_id: author.id) == author.posts
    # => true

Fixes #13506
please use Ruby, not ActiveSupport
Change links to 37signals and svn blog to point to new destinations. [ci skip]
Merge pull request #14717 from vipulnsward/change-37signal-links

Change links to 37signals and svn blog to point to new destinations. [ci skip]
Make the comparison between 'Relation' and 'AssociationRelation'
consistent.