Sequel is a lightweight database access toolkit for Ruby.

* Sequel provides thread safety, connection pooling and a concise DSL
  for constructing database queries and table schemas.
* Sequel also includes a lightweight but comprehensive ORM layer for
  mapping records to Ruby objects and handling associated records.
* Sequel supports advanced database features such as prepared
  statements, bound variables, stored procedures, master/slave
  configurations, and database sharding.
* Sequel makes it easy to deal with multiple records without having
  to break your teeth on SQL.
* Sequel currently has adapters for ADO, Amalgalite, DataObjects,
  DB2, DBI, Firebird, Informix, JDBC, MySQL, ODBC, OpenBase, Oracle,
  PostgreSQL and SQLite3.

Sequel 3.10.0 has been released and should be available on the gem
mirrors.

New Features
------------

* A real one_to_one association was added to Sequel, replacing the
  previous :one_to_one option of the one_to_many association.

  This is a fully backwards incompatible change, any code that uses
  the :one_to_one option of one_to_many will be broken in Sequel
  3.10.0, as that option now raises an exception.  Keeping backwards
  compatibility was not possible, as even the name of the association
  needs to be changed.  Here are the code changes you need to make:

  * The association definition needs to change from one_to_many to
    one_to_one, with no :one_to_one option, and with the association
    name changed from the plural form to the singular form:

      # Before
      Lyric.one_to_many :songs, :one_to_one=>true
      # After
      Lyric.one_to_one :song

  * All usage of the association when eager loading or when getting
    reflections need to use the new singular association name:

      # Before
      Lyric.eager(:songs).all
      Lyric.eager_graph(:songs).all
      Lyric.association_reflection(:songs)
      # After
      Lyric.eager(:song).all
      Lyric.eager_graph(:song).all
      Lyric.association_reflection(:song)

  Any Sequel plugins or extensions that deal with the internals of
  associations need to be made aware of the one_to_one association,
  and how it is different than one_to_many's previous :one_to_one
  option.  Here are some internal changes that may affect you:

  * one_to_one associations are now cached like many_to_one
    associations instead of like one_to_many associations.  So the
    cache includes the associated object or nil, instead of an array.
    Note that this change means that all custom :eager_loader options
    for one_to_one associations need to change to use this new
    caching scheme.

  * The one_to_one association setter method is now handled similarly
    to the many_to_one setter method, instead of using the internal
    one_to_many association add method.

  * Instead of raising an error when multiple rows are returned,
    one_to_one associations now use limit(1) to only return a single
    row.

  There were some other fixes made during these changes:

  * The one_to_one setter now accepts nil to disassociate the record.
    Previously, this raised an error.

  * If the one_to_one association already had a separate object
    associated, and you assigned a different object in the setter
    method, Sequel now disassociates the old object before
    associating the new object, fixing some potential issues if there
    is a UNIQUE constraint on the foreign key column.

  * Using the many_to_one association setter where the reciprocal
    association is a one_to_one association with a currently
    different cached associated object no longer raises an exception.

  * The nested_attributes and association_dependencies plugins
    both now correctly handle one_to_one associations.

  If you need any help migrating, please post on the Sequel Google
  Group or ask in the #sequel IRC channel.

* Both many_to_one and one_to_one associations now use before_set
  and after_set callbacks instead of trying to make the one_to_many
  and many_to_many associations' (before|after)_(add|remove)
  callbacks work.

  This change makes the code simpler, makes writing callbacks easier,
  and no longer requires Sequel to send a query to the database to
  get the currently associated object in the many_to_one association
  setter method (you can still do so manually in a before_set
  callback if you want to).

* Dataset#for_update was added as a default dataset method.
  Previously, it was only supported on PostgreSQL.  It has been
  tested to work on PostgreSQL, MySQL, SQLite (where it is ignored),
  H2, and MSSQL.

* Dataset#lock_style was added as a backbone for Dataset#for_update,
  but allowing you to specify custom lock styles.  These can either
  be symbols recognized by the adapters, or strings which are treated
  as literal SQL.

* Model#lock! was added, which uses Dataset#for_update to lock model
  rows for specific instances.  Combined with the Dataset#for_update,
  Sequel now has an equivalent to ActiveRecord's pessimistic locking
  support.

* A composition plugin was added, given similar functionality as
  ActiveRecord's composed_of.

  The composition plugin allows you to easily define getter and
  setter instance methods for a class where the backing data is
  composed of other getters and decomposed to other setters.

  A simple example of this is when you have a database table with
  separate columns for year, month, and day, but where you want to
  deal with Date objects in your ruby code.  This can be handled
  with:

    Model.composition :date, :mapping=>[:year, :month, :day]

  The :mapping option is optional, but if not used, you need define
  custom composition and decomposition procs via the :composer and
  :decomposer options.

  Note that when using the composition object, you should not modify
  the underlying columns if you are also instantiating the
  composition, as otherwise the composition object values will
  override any underlying columns when the object is saved.

* An rcte_tree plugin was added, which uses recursive common table
  expressions to load all ancestors and descendants in a single
  query.  If your database supports recursive common table
  expressions (PostgreSQL 8.4+, MSSQL 2005+, newer versions of
  Firebird), using recursive common table expressions to load
  all ancestors and descendants is significantly faster than storing
  trees as nested sets and using nested set queries.  Usage:

    Model.plugin :rcte_tree

    # Lazy loading
    model = Model.first
    model.parent
    model.children
    model.ancestors # Populates :parent association as well
    model.descendants # Populates :children association as well

    # Eager loading - also populates the :parent and children
    # associations for all ancestors and descendants
    Model.filter(:id=>[1, 2]).eager(:ancestors, :descendants).all

    # Eager loading children and grandchildren
    Model.filter(:id=>[1, 2]).eager(:descendants=>2).all
    # Eager loading children, grandchildren, and great grandchildren
    Model.filter(:id=>[1, 2]).eager(:descendants=>3).all

* Dataset#first_source_table was added, giving you the unaliased
  version of the table for the first source.

* Add Sequel::BasicObject.remove_methods!, useful on ruby 1.8 if you
  require other libraries after Sequel that add methods to Object.
  For example, if YAML is required after sequel, then the following
  will raise an error:

    DB[:a].filter{x > y}

  because YAML adds the y method to all objects.  Now, you can call
  Sequel::BasicObject.remove_methods!, which will remove those
  methods from Sequel::BasicObject, allowing them to be used as
  intended in the above DSL.

* Sequel associations now accept an :eager_loader_key option, which
  can be useful for associations to specify the column to use for the
  key_hash for custom :eager_loaders.

* A JDBC subadapter for the AS400 database was added.

Other Improvements
------------------

* The one_to_one setter method and the one_to_many and many_to_many
  remove_all methods now apply the association options (such as
  filters) on the appropriate dataset:

    Artist.one_to_many :good_albums, :class=>:Album,
     :conditions=>{:good=>true}
    a = Artist[10]
    a.remove_all_good_albums
    # Before: WHERE artist_id = 10
    # After:  WHERE artist_id = 10 AND good IS TRUE

* Plugin loading now works correctly when the plugin module name
  is the same name as an already defined top level constant.  This
  means that the active_model plugin should now work correctly if
  you require active_model before loading the Sequel plugin.

* The nested_attributes plugin now preserves nested attributes for
  *_to_one associations on validation failures.

* Transactions now work correctly on Oracle when using the JDBC
  adapter.

* Dataset#limit once again works correctly on MSSQL 2000.  It was
  broken in Sequel 3.9.0.

* many_to_one associations now use limit(1) to ensure only one
  record is returned.  If you don't want this (because maybe you
  are using the :eager_graph association option), you need to
  set the :key option to nil and use a custom :dataset option.

* many_to_one and one_to_many associations now work correctly
  with the association :eager option to eagerly load associations
  specified by :eager when lazy loading the association.

* The typecast_on_load plugin now correctly handles
  reloading/refreshing the object, both explicitly and implicitly
  on object creation.

* The schema parser and dumper now return tinyint columns as
  booleans when connecting to mysql using the do adapter, since
  DataObjects now returns the columns as booleans.

* The schema dumper now deals better with unusual or database
  specific primary key types when using the :same_db option.

* On ruby 1.8, Sequel::BasicObject now undefs private methods in
  addition to public and protected methods.  So the following
  code now works as expected:

    DB[:a].filter{x > p} # WHERE x > p

* Sequel.connect with a block now returns the value of the block:

    max_price = Sequel.connect('sqlite://items.db') do |db|
      db[:items].max(:price)
    end

* MSSQL emulated offset support now works correctly when Sequel's
  core extensions are not loaded.

* Sequel::BasicObject now works correctly on rubinius, and almost
  all Sequel specs now pass on rubinius.

* The nested_attributes plugin now uses a better exception message
  no matching associated object is found.

* Sequel now raises a more informative error if you attempt to use
  the native sqlite adapter with the sqlite3 gem instead of the
  sqlite3-ruby gem.

* Multiple complex expressions with the same operator are now
  combined for simpler SQL:

    DB[:a].filter(:a=>1, :b=>2).filter(:c=>3)
    # Before: (((a = 1) AND (b = 2)) AND (c = 3))
    # After:  ((a = 1) AND (b = 2) AND (c = 3))

* The Sequel::Model dataset methods (class methods proxied to the
  model's dataset) and the Sequel::Dataset mutation methods
  (methods that have a ! counterpart to modify the object in place)
  have both been updated to use new dataset methods added in recent
  versions.

Backwards Compatibility
-----------------------

* The :one_to_one option of the one_to_many associations now raises
  an exception.  Please see the section above about the new real
  one_to_one association.

* The change to apply the association options to the one_to_many and
  many_to_many remove_all methods has the potential to break some
  code that uses the remove_all method on associations that use
  association options. This is especially true for many_to_many
  associations, as filters in many_to_many associations will often
  reference columns in the associated table, while the dataset
  used in the remove_all method only contains the join table.  Such
  cases should be handled by manually overriding the _remove_all
  association instance method in the class.  It was determined that
  it was better to issue possibly invalid queries than to issue
  queries that make unexpected modifications.

* Dataset#group_and_count now longer orders the dataset by the count.
  Since it returns a modified dataset, if you want to order the
  dataset, just call order on the returned dataset.

* many_to_one associations now require a working :class option.
  Previously, if you provided a custom :dataset option, a working
  :class option was not required in some cases.

* The MSSQL shared adapter dataset methods switched from using
  the :table_options internal option key to using the :lock internal
  option key.

Thanks,
Jeremy

* {Website}[http://sequel.rubyforge.org]
* {Source code}[http://github.com/jeremyevans/sequel]
* {Blog}[http://sequel.heroku.com]
* {Bug tracking}[http://code.google.com/p/ruby-sequel/issues/list]
* {Google group}[http://groups.google.com/group/sequel-talk]
* {RDoc}[http://sequel.rubyforge.org/rdoc]
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