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    copied!<p>I know this doesn't address the error you're getting, but I'd suggest using three one-to-many relationships instead of one many-to-many relationship.</p> <p>The conventional purpose of <code>has_many :through =&gt; ...</code> (many-to-many) is for when you have something like <code>students</code> and <code>classes</code>. A student can be in <strong>any</strong> number of classes. A class can have <strong>any</strong> number of students. Totally arbitrary numbers on both sides of the relationship.</p> <p>But that isn't your situation here. Your projects can be in exactly one main category, one optional category 1, and one optional category 2. It's a totally different problem and it isn't the problem that <code>has_many :through</code> is designed to solve.</p> <p>I suggest this arrangement:</p> <pre><code>class Project &lt; ActiveRecord::Base belongs_to :main_category, :class_name =&gt; "Category", :foreign_key =&gt; 'main_category_id' belongs_to :optional_category_1, :class_name =&gt; "Category", :foreign_key =&gt; 'optional_category_1_id' belongs_to :optional_category_2, :class_name =&gt; "Category", :foreign_key =&gt; 'optional_category_2_id' end class Category &lt; ActiveRecord::Base has_many :main_category_projects, :class_name =&gt; "Project", :foreign_key =&gt; 'main_category_id' has_many :optional_category_1_projects, :class_name =&gt; "Project", :foreign_key =&gt; 'optional_category_1_id' has_many :optional_category_2_projects, :class_name =&gt; "Project", :foreign_key =&gt; 'optional_category_2_id' end </code></pre> <p>Then you'll be able to do stuff like:</p> <pre><code>my_project.main_category my_category.optional_category_1_projects # etc... </code></pre>
 

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