module Sequel::Plugins::DatasetAssociations

  1. lib/sequel/plugins/dataset_associations.rb

DatasetAssociations allows you to easily use your model associations via datasets. For each association you define, it creates a dataset method for that association that returns a dataset of all objects that are associated to objects in the current dataset. Here's a simple example:

class Artist < Sequel::Model
  plugin :dataset_associations
  one_to_many :albums
# SELECT * FROM albums
# WHERE (albums.artist_id IN (
#   SELECT id FROM artists
#   WHERE ((id >= 1) AND (id <= 100))))

This works for all of the association types that ship with Sequel, including ones implemented in other plugins. Most association options that are supported when eager loading are supported when using a dataset association. However, it will only work for limited associations or *_one associations with orders if the database supports window functions.

As the dataset methods return datasets, you can easily chain the methods to get associated datasets of associated datasets:

Artist.where(id=>1..100).albums.where{name < 'M'}.tags
# SELECT tags.* FROM tags
# WHERE ( IN (
#   SELECT albums_tags.tag_id FROM albums
#   INNER JOIN albums_tags
#     ON (albums_tags.album_id =
#     ((albums.artist_id IN (
#       SELECT id FROM artists
#       WHERE ((id >= 1) AND (id <= 100)))
#     AND
#     (name < 'M')))))

For associations that do JOINs, such as many_to_many, note that the datasets returned by a dataset association method do not do a JOIN by default (they use a subquery that JOINs). This can cause problems when you are doing a select, order, or filter on a column in the joined table. In that case, you should use the :dataset_associations_join option in the association, which will make sure the datasets returned by the dataset association methods also use JOINs, allowing such dataset association methods to work correctly.


# Make all model subclasses create association methods for datasets
Sequel::Model.plugin :dataset_associations

# Make the Album class create association methods for datasets
Album.plugin :dataset_associations