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PORails: Manipulating a Join Table in a Nested Form
 

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    <p>I have two models connected by a join table:</p> <pre><code>class Publication &lt; ActiveRecord::Base attr_accessible :title, :author_attributes, :translator_attributes has_many :publication_contributors has_many :authors, :through =&gt; :publication_contributors, :source =&gt; :contributor, :conditions =&gt; {:publication_contributors =&gt; {:contributor_type =&gt; "Author"}} has_many :translators, :through =&gt; :publication_contributors, :source =&gt; :contributor, :conditions =&gt; {:publication_contributors =&gt; {:contributor_type =&gt; "Translator"}} accepts_nested_attributes_for :authors, :translators end class Contributor &lt; ActiveRecord::Base attr_accessible :name has_many :publications, :through =&gt; :publication_contributors has_many :publication_contributors end class PublicationContributor &lt; ActiveRecord::Base attr_accessible :contributor_type belongs_to :publication belongs_to :contributor end </code></pre> <p>Note that the join table has a third attribute (beside publication_id and contributor_id), which is called contributor_type. This attribute might contain a role such as "Author", "Translator", "Editor", or something else. In my Publication model, I created a pair of associations for two of the more common contributor_types, "Author" and "Translator". These associations work well at retrieving the relevant data, such as with <code>@publication.authors</code>. It sputters when creating these associations through a nested form, however.</p> <p>My form looks something like this:</p> <pre><code>&lt;%= form_for @publication, :remote =&gt; true do |f| %&gt; &lt;%= f.label :title %&gt;: &lt;%= f.text_field :title %&gt; &lt;%= f.fields_for :authors do |builder| %&gt; &lt;%= builder.label :name, "Author" %&gt;: &lt;%= builder.text_field :name %&gt; &lt;% end %&gt; &lt;%= f.fields_for :translators do |builder| %&gt; &lt;%= builder.label :name, "Translator" %&gt;: &lt;%= builder.text_field :name %&gt; &lt;% end %&gt; &lt;%= f.submit %&gt; &lt;% end %&gt; </code></pre> <p>In my controller:</p> <pre><code>def create publication = Publication.create(params[:publication]) end </code></pre> <p>The form renders as expected, but it sputters during the create action. I had hoped that Rails would know to magically assign the proper contributor_type based on the conditions in the Publication associations, such as:</p> <pre><code>has_many :authors, :through =&gt; :publication_contributors, :source =&gt; :contributor, :conditions =&gt; {:publication_contributors =&gt; {:contributor_type =&gt; "Author"}} </code></pre> <p>Unfortunately, it does not. I get this error during creation:</p> <pre><code>Mysql2::Error: Column 'contributor_type' cannot be null </code></pre> <p>This makes me think that my only recourse is to manually assign the contributor_type in a <code>before_create</code> callback, but if that's the case, how do I determine which type to assign? The parameters have the proper type in their name, for instance:</p> <pre><code>publication[authors_attributes][0][name] </code></pre> <p>Is there some way to access that information in the model layer?</p> <p>UPDATE:</p> <p>My <code>new</code> action:</p> <pre><code>def new @publication = Publication.new publication_contributor = @publication.publication_contributors.build contributor = publication_contributor.contributor.build end </code></pre> <p>It throws this error on the final line (the one setting <code>contributor</code>):</p> <pre><code>undefined method `build' for nil:NilClass </code></pre>
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    1. COcan you try adding a hidden field on the form instead?
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    2. COI'm not sure if that would help. The `contributor_type` attribute is in the join table, not in the Contributor model. Since I'm not actually declaring the join table within the form, I don't have access to its attributes, even with a hidden field. Perhaps if there is a way to create the join table within the form, however, this approach could work. I'll look into this. Let me know if you have any other suggestions.
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    3. COyou're right. is it fine if you go through publication_contributor and then contributor?
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