=== Better model associations

The latest release of sequel_model includes a new associations
functionality written by Jeremy Evans which replaces the old relations
code in previous versions. Please note that this version is not
completely backward-compatible and you should therefore upgrade with
caution.

The new implementation supports three kinds of relations: one_to_many,
many_to_one and many_to_many, which correspond to has_many, belongs_to
and has_and_belongs_to_many relations in ActiveRecord. In fact, the
new implementation includes aliases for ActiveRecord assocation macros
and is basically compatible with ActiveRecord conventions. It also
supports DRY implicit class name references. Here's a simple example:

  class Author < Sequel::Model
    has_many :books # equivalent to one_to_many
  end

  class Book < Sequel::Model
    belongs_to :author # equivalent to many_to_one
    has_and_belongs_to_many :categories # equivalent to many_to_many
  end

  class Category < Sequel::Model
    has_and_belongs_to_many :books
  end

These macros will create the following methods:

* Author#books, Author#add_book, Author#remove_book
* Book#author, Book#categories, Book#add_category,
Book#remove_category
* Category#books, Category#add_book, Category#remove_book

Unlike ActiveRecord, one_to_many and many_to_many association methods
return a dataset:

  a = Author[1234]
  a.books.sql #=> 'SELECT * FROM books WHERE (author_id = 1234)'

You can also tell Sequel to cache the association result set and
return it as an array:

  class Author < Sequel::Model
    has_many :books, :cache => true
  end

  Author[1234].books.class #=> Array

You can of course bypass the defaults and specify class names and key
names:

  class Node < Sequel::Model
    belongs_to :parent, :class => Node
    belongs_to :session, :key => :producer_id
  end

Another useful option is :order, which sets the order for the
association dataset:

  class Author < Sequel::Model
    has_many :books, :order => :title
  end

  Author[1234].books.sql #=> 'SELECT * FROM books WHERE (author_id =
1234) ORDER BY title'

More information about associations can be found in the Sequel
documentation.

=== Other changes

* Added configuration file for running specs (#186).

* Changed Database#drop_index to accept fixed arity (#173).

* Changed column definition sql to put UNSIGNED constraint before
unique in order to satisfy MySQL (#171).

* Enhanced MySQL adapter to support load data local infile_, added
compress option for mysql connection by default (#172).

* Fixed bug when inserting hashes in array tuples mode.

* Changed SQLite adapter to catch RuntimeError raised when executing a
statement and raise Error::InvalidStatement with the offending SQL and
error message (#188).

* Fixed Dataset#reverse to not raise for unordered dataset (#189).

* Added Dataset#unordered method and changed #order to remove order if
nil is specified (#190).

* Fixed reversing order of ASC expression (#164).

* Added support for :null => true option when defining table columns
(#192).

* Fixed Symbol#method_missing to accept variable arity (#185).

