# Polymorphic associations. Agile rails book confusion **URL:** \ **Category:** Rails\ **Created:** [June 2, 2007, 6:51am UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758 "2007-06-02T06:51:58Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 6:51am UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/1 "2007-06-02T06:51:58Z")
On page 350 in the PDF it mentions that having multiple tables have a has\_one relationship to the same table cannot work, that is, class Article has\_one :catalog\_entry end class Sound has\_one :catalog\_entry end class Image has\_one :catalog\_entry end I quote, “We can’t possibly arrange to have the foreign key in the catalog entry point back to all three tables” If I understand correctly, it says catalog\_entry cannot contain a reference to all three tables. My question is, why not? If catalog\_entry has two foreign keys, image\_id and sound\_id, wouldn’t all the following calls work? Image.catalog\_entry Sound.catalog\_entry CatalogEntry.image CatalogEntry.sound I cannot see the problem the book suggests is there, what am I missing? ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 10:47am UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/2 "2007-06-02T10:47:34Z")
Michael C. wrote: > On page 350 in the PDF it mentions that having multiple tables have a > has\_one relationship to the same table cannot work, that is, > > class Article > has\_one :catalog\_entry > Image.catalog\_entry > Sound.catalog\_entry > CatalogEntry.image > CatalogEntry.sound > > I cannot see the problem the book suggests is there, what am I missing? You have to use blongs\_to when you use has\_one, that’s why it’s not working 🙂 ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 11:53am UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/3 "2007-06-02T11:53:33Z")
Jamal S. wrote: > You have to use blongs\_to when you use has\_one, that’s why it’s not > working 🙂 > Oh sure, I know, but… class CatalogEntry belongs\_to :image belongs\_to :sound belongs\_to :article end I’ve not used multiple belongs\_to associations in a single class yet, but I can’t see why this wouldn’t work as long as there are foreign keys for each one. In this case the catalog entry would be pointing to all three tables, would it not? ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 12:35pm UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/4 "2007-06-02T12:35:42Z")
Michael C. wrote: > Jamal S. wrote: > > > You have to use blongs\_to when you use has\_one, that’s why it’s not > > working 🙂 > > I’ve not used multiple belongs\_to associations in a single class yet, > but I can’t see why this wouldn’t work as long as there are foreign keys > for each one. > > In this case the catalog entry would be pointing to all three tables, > would it not? Can I see your tables 🙂 class Blog has\_many :comments end class Comments belongs\_to :blog end The comments table MUST have a columns named “blog\_id”, or else the association would not work. ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 6:02pm UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/5 "2007-06-02T18:02:11Z")
I think the goal is to get something like CatalogEntry.resource to accept or return a number of different objects. If polymorphic associations are being used than we could do something like this: CatalogEntry.resource = Image.new or CatalogEntry.resource = Sound.new or CatalogEntry.resource = Video.new Normal associations won’t allow that. Jim On Jun 1, 10:51 pm, Michael C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 10:56pm UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/6 "2007-06-02T22:56:27Z")
Its really the “We cant possibly arrange to have the foreign key in the catalog entry point back to all three tables” part that confused me. As far as I can see, you can point back to all three, you just have to add more foreign keys… But I think I’m starting to see the point, the idea is to point back to either of the three without using more foreign keys. That is, so you can access the resource without necessarily knowing what the resource is. If I’m making sense of this correctly, it would make better sense to use regular associations if the catalog entry is associated with all three classes, but use polymorphic if it can be either of the three. If I have this all backwards, let me know, otherwise thanks 🙂 ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 2, 2007, 6:04pm UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/7 "2007-06-02T18:04:59Z")
Understanding Polymorphic Associations [http://wiki.rubyonrails.org/rails/pages/UnderstandingPolymorphicAssociations](http://wiki.rubyonrails.org/rails/pages/UnderstandingPolymorphicAssociations) You get some examples how it works 🙂 ---
**Author:** ![mcampbell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mcampbell](https://www.ruby-forum.com/u/mcampbell)\ **Post date:** [June 3, 2007, 12:16am UTC](https://www.ruby-forum.com/t/polymorphic-associations-agile-rails-book-confusion/105758/8 "2007-06-03T00:16:49Z")
On 6/2/07, Michael C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > If I’m making sense of this correctly, it would make better sense to use > regular associations if the catalog entry is associated with all three > classes, but use polymorphic if it can be either of the three. If I have > this all backwards, let me know, otherwise thanks 🙂 Nope - looks like you’ve got it. To clear up your initial confusion, when they say “can’t arrange to have the foreign key point back to all three tables”, think of it as “can’t arrange to have _one single_ foreign key point back to all three tables”. Of course you can do it with three foreign keys, but as you now understand, that isn’t the point of polymorphic associations. – Bill K. [http://bkocik.net](http://bkocik.net)