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Cheat Sheet:
Options for has_many :class_name - specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_many :products will by default be linked to the Product class, but if the real class name is SpecialProduct, you’ll have to specify it with this option. :conditions - specify the conditions that the associated objects must meet in order to be included as a "WHERE" sql fragment, such as "price > 5 AND name LIKE ‘B%’". :order - specify the order in which the associated objects are returned as a "ORDER BY" sql fragment, such as "last_name, first_name DESC" :group - specify the attribute by which the associated objects are returned as a "GROUP BY" sql fragment, such as "category" :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_many association will use "person_id" as the default foreign_key. :dependent - if set to :destroy all the associated objects are destroyed alongside this object by calling their destroy method. If set to :delete_all all associated objects are deleted without calling their destroy method. If set to :nullify all associated objects’ foreign keys are set to NULL without calling their save callbacks. NOTE: :dependent => true is deprecated and has been replaced with :dependent => :destroy. May not be set if :exclusively_dependent is also set. :exclusively_dependent - Deprecated; equivalent to :dependent => :delete_all. If set to true all the associated object are deleted in one SQL statement without having their before_destroy callback run. This should only be used on associations that depend solely on this class and don’t need to do any clean-up in before_destroy. The upside is that it’s much faster, especially if there’s a counter_cache involved. May not be set if :dependent is also set. :finder_sql - specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depend on multiple tables. Note: When this option is used, find_in_collection is not added. :counter_sql - specify a complete SQL statement to fetch the size of the association. If +:finder_sql+ is specified but +:counter_sql+, +:counter_sql+ will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM. :extend - specify a named module for extending the proxy, see "Association extensions". :include - specify second-order associations that should be eager loaded when the collection is loaded. :group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause. :limit: An integer determining the limit on the number of rows that should be returned. :offset: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows. :select: By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns. :as: Specifies a polymorphic interface (See belongs_to). :through: Specifies a Join Model to perform the query through. Options for :class_name and :foreign_key are ignored, as the association uses the source reflection. You can only use a :through query through a belongs_to or has_many association. :source: Specifies the source association name used by has_many :through queries. Only use it if the name cannot be inferred from the association. has_many :subscribers, :through => :subscriptions will look for either +:subscribers+ or +:subscriber+ on Subscription, unless a +:source+ is given.

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