Last Update: 2013-11-10 14:46:11 -0800

The Most Powerful and Flexible Associations of Any Ruby ORM

Sequel can now support any association type supported by ActiveRecord, and many association types ActiveRecord doesn’t support.

Association callbacks (:before_add, :after_add, :before_remove, :after_remove) have been added, and work for all association types. Each of the callback options can be a Symbol specifying an instance method that takes one argument (the associated object), or a Proc that takes two arguments (the current object and the associated object), or an array of Symbols and Procs. Additionally, an :after_load callback is available, which is running after loading the associated record(s) from the database.

Association extensions are now supported:

class FindOrCreate
  def find_or_create(vals)
    first(vals) || create(vals)
class Author < Sequel::Model
  one_to_many :authorships, :extend=>FindOrCreate

Sequel has been able to support most has_many :through style associations since 1.3, via many_to_many (since it doesn’t break on join tables that are also model tables, unlike ActiveRecord’s has_and_belongs_to_many). Now it can also support has_many :through style associations where it goes through a has_many association.

Sequel can now support polymorphic associations. Polymorphic associations are really a design flaw, so Sequel doesn’t support them directly, but the tools that Sequel gives you make them pretty easy to implement.

Sequel can also support associations that ActiveRecord does not. For example, a belongs_to association where the column referenced in the associated table is not the primary key, an association that depends on multiple columns in each table, or even situations where the association has a column in the primary table that can be referenced by any of multiple columns in a second table that has a has_one style association with the table you want to associate with.

Some of those associations can be supported for a single object using custom SQL in ActiveRecord, but none are supported when eager loading or allow further filtering.

Not only can all of these cases be supported with Sequel::Model, all can be supported with eager loading, and can allow for further filtering. See sequel.rubyforge.org/files/sequel/doc/advanced_associations_rdoc.html for details and example code for all association types covered above.

There have also been many additional options added for controlling eager loading via eager_graph. Every part of the SQL JOINs can now be controlled via one of the options, so you can use JOIN USING, NATURAL JOIN, or arbitrary JOIN ON conditions.

Finally, just to show off the power that Sequel gives you when eager loading, here is example code that will eagerly load all descendants and ancestors in a tree structure, without knowing the depth of the tree:

class Node < Sequel::Model
  set_schema do
    primary_key :id
    foreign_key :parent_id, :nodes

  many_to_one :parent
  one_to_many :children, :key=>:parent_id

  # Only useful when eager loading
  many_to_one :ancestors, :eager_loader=>(proc do |key_hash, nodes,


# Handle cases where the root node has the same parent_id as


# and also when it is NULL
non_root_nodes = nodes.reject do |n|
  if [nil, n.pk].include?(n.parent_id)
    # Make sure root nodes have their parent association set to


    n.associations[:parent] = nil
unless non_root_nodes.empty?
  id_map = {}
  # Create an map of parent_ids to nodes that have that parent id
  non_root_nodes.each{|n| (id_map[n.parent_id] ||= []) << n}
  # Doesn't cause an infinte loop, because when only the root node
  # is left, this is not called.

do |node|

      # Populate the parent association for each node
      id_map[node.pk].each{|n| n.associations[:parent] = node}
many_to_one :descendants, :eager_loader=>(proc do |key_hash, nodes,


id_map = {}
nodes.each do |n|
  # Initialize an empty array of child associations for each

parent node

  n.associations[:children] = []
  # Populate identity map of nodes
  id_map[n.pk] = n
# Doesn't cause an infinite loop, because the :eager_loader is not


# if no records are returned.  Exclude id = parent_id to avoid

infinite loop

# if the root note is one of the returned records and it has

parent_id = id

# instead of parent_id = NULL.

do |node|

# Get the parent from the identity map
parent = id_map[node.parent_id]
# Set the child's parent association to the parent
node.associations[:parent] = parent
# Add the child association to the array of children in the


      parent.associations[:children] << node

nodes = Node.filter(:id < 10).eager(:ancestors, :descendants).all

New Adapter Features

  • PostgreSQL bytea fields are now fully supported.

  • The PostgreSQL adapter now uses the safer connection-specific string escaping if you are using ruby-pg.

  • The SQLite adapter supports drop_column and add_index.

  • You can now use URL parameters in the connection string, enabling you to connect to PostgreSQL via a socket using postgres://user:password@blah/database?host=/tmp

Other New Features

  • Dataset#graph now takes a block which it passes to join_table.

  • Symbol#identifier has been added, which can be used if another library defines the same operator(s) on Symbol that Sequel defines.

  • Filter blocks now yield a VirtualRow instance, which can yield Identifiers, QualifiedIdentifiers, or Functions. Like Symbol#identifier, this is useful if another library defines the same operator(s) on Symbol that Sequel defines.

  • You can now call Model.to_hash to get an identity map for all rows (before this required Model.dataset.to_hash).

  • A model that can get it’s column information from the schema will set it in the dataset, potentially saving many queries.

  • Model.validates_presence_of now works correctly for boolean columns.

Notable Bug Fixes

  • Caching now works with Model subclasses.

  • Model validation methods now work with source reloading.

  • The PostgreSQL adapter no longer raises an Error if you try to insert a record with the primary key already specified.

  • Sequel no longer messes with the native MySQL adapter, so you can use Sequel and ActiveRecord with MySQL in the same process.

  • Dataset#count now works correctly for limited dataset.

  • PostgreSQL Database#transaction method yields a connection, similar to the other adapters.

  • Using a hash argument in distinct, order, or group is treated as an expression instead of a column alias.

  • Cloned datasets no longer ignore the existing columns unless it is necessary.

  • The :quote_identifiers and :single_threaded Database options now work correctly.

Backwards Incompatible Changes

  • ParseTree support, deprecated in 2.1.0, has been removed in 2.2.0. You should use the expression filter syntax instead, perferably without the block (though it can be used inside a block as well). This usually involves the following types of changes:

    filter{:x == :y} => filter(:x => :y)
    filter{:x << :y} => filter(:x => :y)
    filter{:x && :y} => filter(:x & :y) # Don't forget about change
    filter{:x || :y} => filter(:x | :y) #  in operator precedence
    filter{:x.like?('%blah%')} => filter(:x.like('%blah%'))
    filter do => filter((:x > 1) & (:y < 2))
      :x > 1
      :y < 2
  • Attempts to save an invalid Model instance will raise an error by default. To revert to returning a nil value, use:

    Sequel::Model.raise_on_save_failure = false # Global
    Album.raise_on_save_failure = false # Class
    album = Album.new
    album.raise_on_save_failure = false # Instance

    Note that before, save would return false where now it returns nil if you disable raising on save failure.

  • Dataset#update no longer takes a block, as it’s use of the block depended on ParseTree. With the introduction of the expression syntax in 2.0.0, it’s no longer necessary. You should use a hash with an expression as the value instead:

    DB[:table].update(:column=>:column + 1)
  • validates_presence of now considers false as present instead of absent. This is so it works with boolean columns.

  • Dataset#graph ignores any previously selected columns when it is called for the first time.

  • Dataset#columns ignores any filtering, ordering, or distinct clauses. This shouldn’t cause issues unless you were using SQL functions with side effects and expecting them to be called when columns was called (unlikely at best).

One significant point of note is that the 2.2.0 release will be the last release with both a sequel_core and sequel gem. Starting with 2.3.0 they will be combined into one sequel gem. You will still be able to get just the sequel_core part by requiring ‘sequel_core’, but they will be packaged together.