Fix eagerly loading associations without primary keys
Change JoinPart test from an integration to a unit test
Fixed validation issue in Upgrading Ruby on Rails guide [ci skip]

In chapter 2.4 there was an underscore at the start of the attr_readonly method name,
this underscore was interpreted as the opening of the <em> tag but it wasn't closed.

Worst, the underscore in self.class.serialized_attributes (end of paragraph)
was interpreted as the closing of the <em> tag and not as the name of a method.
Fixed markup in Upgrading RoR guide  [ci skip]

There are two changes:
- Using <ruby> syntax for unicode character routes example instead of
  <tt> since it causes single quotes to be transformed into inverted
  commas. This makes this required change more eye-catching.
- Using <tt> for the j alias and related helpers as written in the
  documentation guidelines.
Fix example code: use tasks instead of comments
Merge pull request #7504 from behrends/patch-1

Fix example code: use tasks instead of comments [ci skip]
Merge pull request #7502 from xoen/fixed-upgrading-guide-markup

Fixed markup in Upgrading RoR guide [ci skip]
Underscore in _attr_readonly method name is back (Upgrading RoR guide) [ci skip]
Fixed validation issues related to taken IDs in AR Querying guide [ci skip]
Fixed validation issues in the "Configuring Rails Applications" guide [ci skip]

erb tag wasn't closed well
Fixed validation issue related to taken ID in Layout and Rendering guide [ci skip]
Fixed validation issue and titles in Migrations guide [ci skip]

Changed the title of the 3.5 chapter from "Using the change Method" to
"When to Use the change Method" since the chapter tells the migration
definitions supported by the change method. This fix the duplicate ID in the
document.

I've also changed some of the titles to respect the Guides guidelines.
Fix indenentation in comment to stop the end of comment from appearing in a <pre> block.
Add section about tagged logging

Tagged logging for which Rails has support since [3.2](http://guides.rubyonrails.org/3_2_release_notes.html#tagged-logging).
It is trivial & useful but unless somebody explicitly searches API docs, he/she may not know about it. 

Hence, I added it in guides. 
set the configured #inheritance_column on #become (#7503)

I had to create a new table because I needed an STI table,
which does not have both a "type" and a "custom_type"

the test fails with:
  1) Error:
test_alt_becomes_works_with_sti(InheritanceTest):
NoMethodError: undefined method `type=' for #<Cabbage id: 1, name: "my cucumber", custom_type: "Cucumber">
    /Users/username/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:432:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:100:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/persistence.rb:165:in `becomes'
    test/cases/inheritance_test.rb:134:in `test_becomes_works_with_sti'
    test/cases/inheritance_test.rb:140:in `test_alt_becomes_works_with_sti'
Merge pull request #7506 from senny/issue_7503

#becomes works with configured inheritance_column
Allow routing concerns to accept a callable

This allows us to make alterations to the generated routes based on the
scope of the current mapper, and otherwise allows us to move larger
blocks of concerns out of the routes file, altogether.
Fix concerns not executing block in mapper

Also, add documentation for alternate usage.
Make enhanced routing Concerns more tell-don't-ask
rewrite inheritance tests with a custom inheritance_column

previously the tests with and without a custom `inheritance_column`
used the same models. Since the model then has both fields this can lead
to false positives.