DirtyModel uses a hash to keep track of any changes made to attributes
of an instance. When using the attribute_will_change! method, you must
supply a string and not a symbol or the *_changed? method will break
(because it is looking for the attribute name as a string in the keys
of the underlying hash). To remedy this, I simply made the underlying
hash a HashWithIndifferentAccess so it won't matter if you supply
the attribute name as a symbol or string to attribute_will_change!.
Squashed commit of the following:

commit 02d3b059608c30e98136fde78bc710928f080566
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 30 15:31:39 2013 -0700

    habtm works in terms of hm:t

commit 71ac336bbb41f5047a4ee307883a95eca7195742
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 30 15:27:07 2013 -0700

    passing before_add callbacks

commit d846a7bf9872a79c3aa8082917abe806278fa159
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 17:57:53 2013 -0700

    reducing diff against master

commit 96bd97de47d61a71c368ae367bc59a2dbec3c9ab
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 17:52:35 2013 -0700

    fixing more tests

commit 0620399fc231df87c5f08664db1c37e5c1fa5a05
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 17:32:52 2013 -0700

    self-referential tables in hm:t are allowed to have the same pk

commit 48eb90e27921d10b6ba3e400ab2c784ed75d5ec4
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 17:03:19 2013 -0700

    translating more options

commit 5cace7b2cb546fd6b096543bfc49c4b7197ad21a
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 17:01:02 2013 -0700

    handling more hm:t cases

commit 69985ca2cabff2c3f58f5d0a7eb12d7b414c1a01
Merge: d417ec8 3e0a60e
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 27 16:57:12 2013 -0700

    Merge branch 'master' into rmhbtm

    * master: (21 commits)
      adding a test to demonstrate how to use STI subclasses on the far right side of a hm:t association along with preloading.
      Fixed grammatical typo in configuring.md guide.
      Getting Started Guide: Fix code container in Chapter 5.2 [ci-skip]
      Getting Started Guide: Update sentence in Chapter 5.8 [ci-skip]
      Add new line after create action on CommentsController code example
      Adds template dependencies rake task from cache_digests gem.
      [ci skip] Update scaffold output and change some words.
      [ci skip] escape unintended url in docs
      Getting Started Guide: post.rb -> Post
      [ci skip] Add missing periods and update link name and some wording.
      quote `false` reference in querying guide.
      Getting Started Guide: update link_to string argument to use single-quote mark, following document style [ci skip]
      Fix small typo in docs
      changelog entry
      fix .find when inverse is set
      update changelog for #12359
      Make sure inverse_of is visible on the has_many callbacks
      Getting Started Guide: posts_controller -> PostsController [ci skip]
      [ci skip] Correct the explanation of the example for find_or_create_by when used with create_with in ActiveRecord Querying guide
      added "id: false" to HABTM join table example
      ...

commit d417ec82e8f83c32124d1c1a19824d023cfdf015
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 25 16:49:08 2013 -0700

    another case is passing

commit 5c68280500962e4b2b6819dd863ebe8b398e5834
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 25 16:21:22 2013 -0700

    this seems to work

commit e458c5e55c04a2444e96aca1ff192be42bc4ce7f
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 25 16:13:07 2013 -0700

    add another case

commit fc6203b0d49c847b8efb1cc33d358897625f2115
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 25 15:51:45 2013 -0700

    delete center records on habtm

commit 9af5156098f6bc8f8ce8eb559a51137960b4938b
Merge: 3a60b03 e2fd64f
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 25 11:33:13 2013 -0700

    Merge branch 'master' into rmhbtm

    * master: (100 commits)
      remove initialize method
      extract association resetting to a method
      hash insertion order doesn't matter anymore, so only loop over the owners once
      always populate the preloaded records instance variable so we can remove the @associated_records_by_owner ivar
      keep preloaded records in a list rather than extract from a hash
      Getting Started Guide: Hello Rails! -> Hello, Rails! and wrap code tag
      push slice loading to it's own method so we can remove the type casting code
      Add CHANGELOG entry for #12344
      Add regression test to #12343
      Fix typo in number_to_human docs: you -> your
      guarantee that `klass` is not nil inside the preloader objects
      [Documentation] Add a missing validation to I18n docs
      Use the given name in html_options for the hidden field in collection_check_boxes
      eliminate unused ivar
      all records have a preloaded, so eliminate that conditional
      eliminate the `loaded?` conditional
      push preloaded test up to the factory method so we can eliminate conditionals from the individual preloaded classes
      assign_attributes should return if argument is blank.
      No need the else clause
      Use join to concat the both side of the AST
      ...

commit 3a60b038a40532397b6c204dfb09d6d43a9336ac
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 17:50:11 2013 -0700

    start with a clean slate

commit f30d3631af11ea6144d3ae7a068a7c0072e93a82
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 17:30:59 2013 -0700

    make sure the class name goes on the rhs belongs_to

commit f7516b724014504ddb2e706fea1b5438dc5332c3
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 16:31:40 2013 -0700

    remove unused variable

commit 61ffc5b9854dc2fe83ee502b17ba8028270ff8a7
Merge: 6cf41cd 460eb83
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 16:25:17 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      support objects with blank string primary keys
      ActiveRecord::Base#<=> has been removed.  Primary keys may not be in order, or even be numbers, so sorting by id doesn't make sense.  Please use `sort_by` and specify the attribute you wish to sort with.  For example, change:
      do what the superclass does in the case that objects do not match

commit 6cf41cd98a82e6f4fe6d868ad323df3d72a9748f
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 15:06:28 2013 -0700

    heating up habtm cache

commit d7f6c3aa491f27ba71fdb2b9d0b9d1780664f4dc
Merge: c68c904 56bfd8a
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 18 14:07:51 2013 -0700

    Merge branch 'master' into rmhbtm

    * master: (58 commits)
      Fix an issue where router can't recognize downcased url encoding path.
      There's no need to do this
      Remove tzinfo dependency from Action Pack
      [ci skip] Improve readability of 4.3's NOTE in migration.md.
      Removes unused code related to DatabaseTasks.
      [ci skip] Consistency wording of 9.6 in form_helpers.md
      [ci skip] Update plugins.md
      Removing ActiveSupport::Concern, it's not needed
      Fixing comment typo in ActionController::Base
      Don't require using application_name before options
      Collapse where constraints to one where constraint
      Custom flash should be defined only for the class that defines it and it's subclasses.
      Fix typos: the indefinite articles(a -> an)
      Missing destroy command
      Update 3_2_release_notes.md
      Add CHANGELOG entry for #11698
      Add CHANGELOG entry for #12149
      Use the Rails binary when generating task
      Remove unnecessary loop
      "generates" applies to "collection radio" so it should be singular
      ...

commit c68c904866ef9562c3bd9b54574206a416184414
Merge: 0f5d8e0 71cf717
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 11 17:45:53 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      these are not real developer objects, so counting them doesn't make sense.  Let's load the object to ensure it's an array and count the array.
      Remove conditional adding a new method
      Fix inverted conditional
      Remove invalid comment
      Check if the SQL is not a prepared statement
      Whitespaces
      Revert "Add meta tag with charset information to application layout."
      Avoid empty transaction from setting has_one association on new record.
      Reduce Duration#inspect to a single series of transformations
      Relation#merge should not lose readonly(false) flag.
      Reduce allocations when extracting AR models
      Perf: avoid dupes add fallback logic for coders

commit 0f5d8e0febd3128cf4121ff36f1764b9284d9f7d
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 11 11:43:29 2013 -0700

    everything works with extensions

commit d003c103b5908fb3a6427f39bddd1748ef2c2576
Merge: 5768c38 7e0cac1
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Sep 11 11:23:59 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      fix deleting join models with no pk
      remove sentence err
      [ci skip] 'previous version of Rails' is gramatically incorrect
      Add meta tag with charset information to application layout.
      add a comment for sanity of other people to come

commit 5768c38d53fd66a97814faaea8e07c70722b310f
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Tue Sep 10 15:11:05 2013 -0700

    habtms with a scope seem to be working

commit 5ee9108d95c544d4befd682a72139383d0780d68
Merge: d5478e6 e64b5da
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Tue Sep 10 11:55:48 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      ask the association for records rather than calling `send`

commit d5478e64bbf80337ec35462368edabb373feeb74
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Tue Sep 10 11:54:39 2013 -0700

    ask the association for records rather than calling `send`

commit 93020bc1ad51363c3f400370f91c9494690dcea8
Merge: 11b3d5f d68419a
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Tue Sep 10 11:30:03 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      Use Ruby 2.0 caller_locations instead of caller if available
      Update Rails 3.2.x guide link [ci skip]
      Be sure to restore the default I18n.locale after changed its value in a test
      Fixes typo in Object#try!
      More unused associations in AR test models
      :scissors: [ci skip]
      change function def self.table_name to self.table_name
      Clean up unused associations in AR test model
      Reset ActionView::Base.logger instead of AC::Base.logger
      Refactor handling of action normalization
      Don't mutate the Base settings by merge!ing the given value
      Make AC standalone rendering work
      use assert_empty in activemodel conditional validation test cases
      Removed unused modules and classes
      Removed unnecessary require
      Remove helper fixtures not used in any test
      Back AV description in gemspec
      Fix order dependent test
      grab executable from rubygems
      Fixed API task file

commit 11b3d5fa45b57fc4e7dddb09be583498b120b185
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 16:19:48 2013 -0700

    change query count since we are using hm:t associations

commit f59daebedab3ed13f31c99244ff71a4a5d6e554b
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 15:35:07 2013 -0700

    delegate compute_type to a real AR class

commit c84a40d2ed76e5843b994c5a2b9e29ced3816511
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 15:29:51 2013 -0700

    define callbacks with the builder

commit d08b1b6b3efc9ed8b0d5476892f048fbffb39e40
Merge: acebec1 0c5d0be
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 15:17:40 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      let the class cache object clean up user input
      make @bitsweat happy. :heart:

commit acebec128e108ac2b4855e540d8764629670cb83
Merge: 2de68a4 e1cbd42
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 14:47:30 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      stop using deprecated api in the tests

commit 2de68a464641f76067743957f889ac87dff395a0
Merge: 7504df9 1385ae1
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 14:34:37 2013 -0700

    Merge branch 'master' into rmhbtm

    * master:
      Remove BasicRendering tests
      Remove remaining coupling with AV in MimeResponds
      Remove BasicRendering and remove template functionality from AbsC::Rendering
      Improves a sentence in guides/security
      [ci skip] Change link name of Rails i18n wiki.
      Typo in Changelog.
      Fix fixtures regression that required table names to map to classes only, not class names
      Use MiniTest::Unit::TestCase instead of Minitest::Test
      Use Ruby on Rails Coding Conventions for code examples in the guides

commit 7504df92f21ed3d5da7c2760d760f026728ed04d
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Mon Sep 9 14:34:02 2013 -0700

    fake class name should be a valid class name

commit 6609620ea86dc0fb7c4bbfb0db950f6e3fc75b56
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:52:03 2013 -0700

    move another habtm

commit 2c95a36e2c3dfe92f2930f3cca44bc4452732a23
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:50:18 2013 -0700

    use the habtm name to generate the rhs name on the join model

commit bd963f720b1db19b0bec186bc33bef9203d8b011
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:47:29 2013 -0700

    don't hardcode the name

commit 461759e2caf66f23dca4eff988648bf769a2b533
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:45:27 2013 -0700

    we do not need to specify the fk

commit 9c223f01db6e36adbb7570e2aa1bcaec1d142c87
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:44:08 2013 -0700

    just call the left side :left_side

commit 5661622a82154eff877fe0993bfffad13dacad7a
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Fri Sep 6 15:40:36 2013 -0700

    initial habtm implementation is working
only pass options to the habtm builder
decouple the HABTM builder
unify `Fixes` notes in AR changelog. [ci skip].
Added activemodel as a explicit dependency

As we have other dependency explicitly here
it's fine to add activemodel as well
Merge pull request #12426 from arunagw/added_activemodel_in_railsgemspec

Added activemodel as a explicit dependency
define the join model without using closures on the methods
get the left and right association names from the join model
move habtm translation to a builder class
decouple define_callback from the instance
name is on the reflection, so just use the reflection
Adds the Rails maintenance policy to the Guides

[ci skip]
push constructable? on to the reflection
association builder is no longer needed for counter cache, so remove it
Running all isolated test for actionview

1. fix name for activerecord directory
2. Added actionpack directory to run tests
Merge pull request #12428 from arunagw/running_isolated_test_av

Running all isolated test for actionview
Merge pull request #12171 from matiaskorhonen/adds-maintenance-policy-to-guides

Adds the Rails maintenance policy to the Guides
use the information on the reflection to determine whether callbacks should be added
valid_options doesn't depend on the instance, so push it to the class