# Help With has\_many and belongs\_to :through assocaitions

**URL:** <https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474>\
**Category:** rubyonrails-talk\
**Created:** [October 23, 2006, 8:14pm UTC](https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474 "2006-10-23T20:14:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![markdodwell](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@markdodwell](https://discuss.rubyonrails.org/u/markdodwell)\
**Post date:** [October 23, 2006, 8:14pm UTC](https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474/1 "2006-10-23T20:14:25Z")

</div>

Hi all, I am having problems with some reasonably complex assocaitions involving has\_many and belongs\_to with the :through option.

I have three models:

class Site \< ActiveRecord::Base &nbsp;&nbsp;has\_many :site\_page\_relationships &nbsp;&nbsp;has\_many :pages, :through =\> :pagesite\_page\_relationships\_relationships, :source =\> :child\_page end

class PageRelationship \< ActiveRecord::Base &nbsp;&nbsp;belongs\_to :site &nbsp;&nbsp;belongs\_to :parent\_page, :foreign\_key =\> :parent\_page\_id, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> :page &nbsp;&nbsp;belongs\_to :child\_page, :foreign\_key =\> :child\_page\_id, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> :page end

class Page \< ActiveRecord::Base &nbsp;&nbsp;has\_many :page\_relationships, :foreign\_key =\> :child\_page\_id &nbsp;&nbsp;has\_many :child\_pages, :through =\> :page\_relationships, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:source =\> :child\_page &nbsp;&nbsp;has\_many :parent\_pages, :through =\> :page\_relationships, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:source =\> :parent\_page end

The idea being that you can have multipe sites each with their own sitetree, and the ability to share pages between sites but in different positions in the site tree.

When I try the following in 'script/console':

> > page1 = Page.new

=\> #\<Page:0x279bb60 @attributes={}, @new\_record=true\>

> > page1.save

=\> true

> > page2 = Page.new

=\> #\<Page:0x27944b4 @attributes={}, @new\_record=true\>

> > page2.save

=\> true

> > site = Site.new

=\> #\<Site:0x278e208 @attributes={"name"=\>nil, "updated\_at"=\>nil, "url"=\>nil, "description"=\>nil, "created\_at"=\>nil}, @new\_record=true\>

> > site.save

=\> true

> > rel1 = PageRelationship.new(:parent\_page =\> page1, :child\_page =\> page2, :site =\> site)

TypeError: can't convert Symbol into String &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active\_record/base.rb:1003:in `type_name_with_module' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/base.rb:1242:in `compute\_type' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active\_record/reflection.rb:112:in `klass' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/associations/association_proxy.rb:133:in `raise\_on\_type\_mismatch' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active\_record/associations/belongs\_to\_association.rb:22:in `replace' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/associations.rb:850:in `child\_page=' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active\_record/base.rb:1510:in `attributes=' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/base.rb:1509:in `attributes=' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active\_record/base.rb:1354:in `initialize_without_callbacks' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/callbacks.rb:236:in `initialize' &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from (irb):11

You can see that I get this error? It's really bugging me, I can't figure out why. Any suggestions/ideas would be much appreciated.

Many Thanks

Mark Dodwell

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<div class="post-metadata">

**Author:** ![Bala\_Paranj](https://avatars.discourse-cdn.com/v4/letter/b/41988e/32.png) [@Bala\_Paranj](https://discuss.rubyonrails.org/u/Bala_Paranj)\
**Post date:** [October 23, 2006, 8:31pm UTC](https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474/2 "2006-10-23T20:31:38Z")

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> class Site \< ActiveRecord::Base has\_many :site\_page\_relationships has\_many :pages, :through =\>
> 
> :pagesite\_page\_relationships\_relationships, :source =\> :child\_page end
> 
> class PageRelationship \< ActiveRecord::Base belongs\_to :site belongs\_to :parent\_page, :foreign\_key =\> :parent\_page\_id,
> 
> ```
> :class_name => :page
> 
> ```
> 
> belongs\_to :child\_page, :foreign\_key =\> :child\_page\_id, :class\_name =\> :page end
> 
> class Page \< ActiveRecord::Base has\_many :page\_relationships, :foreign\_key =\> :child\_page\_id
> 
> has\_many :child\_pages, :through =\> :page\_relationships, :source =\> :child\_page has\_many :parent\_pages, :through =\> :page\_relationships, :source =\> :parent\_page end
> 
> The idea being that you can have multipe sites each with their own
> 
> sitetree, and the ability to share pages between sites but in different positions in the site tree.

If I understood your domain, it is like :

Site 1 -----\> SiteTree 1

Site 2 ------\> SiteTree 2

and so on.

Then SiteTree has many Pages. So it looks like you have one-to-one relationship between Site and SiteTree. SiteTree and Pages many to many relationship.

If that is the case then your model should break the many-to-many relationship between SiteTree and Page classes.

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<div class="post-metadata">

**Author:** ![Bala\_Paranj](https://avatars.discourse-cdn.com/v4/letter/b/41988e/32.png) [@Bala\_Paranj](https://discuss.rubyonrails.org/u/Bala_Paranj)\
**Post date:** [October 23, 2006, 9:02pm UTC](https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474/3 "2006-10-23T21:02:10Z")

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> Your Page model doesn’t have the right associations for a self-referential hmt relationship. You need two different has\_many
> 
> assocs for the parent and child refs to the join model. This is like having different aliases for your joins in SQL. Take a look at my posting on this:
> 
> [http://blog.hasmanythrough.com/articles/2006/04/21/self-referential-through](http://blog.hasmanythrough.com/articles/2006/04/21/self-referential-through)
> 
> Also, your Site class has some weird named assocs. That might be the source of your error. Remember, if the model name can’t be inferred from
> 
> the assoc name, you need to specify it with options.

Edge (n) \<-------------------\>(1) Node -----

^\_\_\_\_\_|

Is the diagram correct? Node has the self-reference and you have one to many relationship between the Node and the Edges.

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<div class="post-metadata">

**Author:** ![markdodwell](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@markdodwell](https://discuss.rubyonrails.org/u/markdodwell)\
**Post date:** [October 23, 2006, 11:33pm UTC](https://discuss.rubyonrails.org/t/help-with-has-many-and-belongs-to-through-assocaitions/3474/4 "2006-10-23T23:33:31Z")

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Many thanks for all your help, I have now managed to fix it. After reading John's post ' Self-referential has\_many :through associations' I sorted it.

Great to get such a quick response from people. First time I posted a question here! Was very happy indeed. My new models which work are as follows:

class Site \< ActiveRecord::Base &nbsp;&nbsp;has\_and\_belongs\_to\_many :users &nbsp;&nbsp;has\_many :page\_relationships &nbsp;&nbsp;has\_many :pages, :through =\> :page\_relationships, :source =\> :child\_page end

class PageRelationship \< ActiveRecord::Base &nbsp;&nbsp;belongs\_to :site &nbsp;&nbsp;belongs\_to :parent\_page, :foreign\_key =\> 'parent\_page\_id', &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> 'Page' &nbsp;&nbsp;belongs\_to :child\_page, :foreign\_key =\> 'child\_page\_id', &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> 'Page' end

class Page \< ActiveRecord::Base &nbsp;&nbsp;has\_many :page\_relationships\_as\_child, :foreign\_key =\> 'child\_page\_id', &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> 'PageRelationship' &nbsp;&nbsp;has\_many :page\_relationships\_as\_parent, :foreign\_key =\> 'parent\_page\_id', &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;:class\_name =\> 'PageRelationship' &nbsp;&nbsp;has\_many :child\_pages, :through =\> :page\_relationships\_as\_child &nbsp;&nbsp;has\_many :parent\_pages, :through =\> :page\_relationships\_as\_parent end

Cheers again, Mark