Need help to understand :source_type option of has_one/has_many through of Rails

Question

In Rails 3.1, the documentation says

"4.2.2.13 :source_type

The :source_type option specifies the source association type for a has_one :through association that proceeds through a polymorphic association. "

I just read :source explanation but still dont get what source_type is used for?

Answer 1

:source_type deals with associations that are polymorphic. That is to say, if you have a relationship like this:

class Tag < ActiveRecord::Base   has_many :taggings, :dependent => :destroy   has_many :books, :through => :taggings, :source => :taggable, :source_type => "Book"   has_many :movies, :through => :taggings, :source => :taggable, :source_type => "Movie" end  class Tagging < ActiveRecord::Base   belongs_to :taggable, :polymorphic => true   belongs_to :tag end  class Book < ActiveRecord::Base   has_many :taggings, :as => :taggable   has_many :tags, :through => :taggings end  class Movie < ActiveRecord::Base   has_many :taggings, :as => :taggable   has_many :tags, :through => :taggings end 

Then the source type allows you to make queries like this:

"Find me all of the books that have been tagged with the tag named 'Fun'"

tag = tag.find_by_name('Fun') tag.books 

Without source type, you wouldn't be able to do that, you could only get a collection of objects that were tagged with 'Fun'. If you only specificed source, it wouldn't know which kind of class the objects were, so you it wouldn't know which table in the DB to pull from. The source_type Informs it of which type of object you are trying to retreive.

This is taken from this blog post: http://www.brentmc79.com/posts/polymorphic-many-to-many-associations-in-rails

Hope it helps.

Answer 2

☝🏼 this message is about the post above (I don't have enough reputation yet to add this as comment...)

Thanks @TheDelChop for that simple and perfect use case. I just propose to complete your perfect explanation with this simple schema which describe your user story. Just in case of. Thank you !