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    <p><code>has_many</code> accepts a block that can define/override methods for the association. This will allow you to create a custom method for <code>&lt;&lt;</code>. I've created a small example for you, you could create build in a similar fashion.</p> <pre class="lang-rb prettyprint-override"><code># Project.rb has_many :developers, :through =&gt; :project_users, :source =&gt; :user, :conditions =&gt; "project_users.role = 'developer'" do def &lt;&lt;(developer) proxy_owner.project_users.create(:role =&gt; 'developer', :user =&gt; developer) end end </code></pre> <p>Now you can add a new developer to your your project with: <code>@project.developers &lt;&lt; @user</code> as requested. <code>@project.developers</code> gives you all the developers. </p> <p>If you have a lot of roles, it might be useful to create these has_many statements dynamically. </p> <pre class="lang-rb prettyprint-override"><code># Project.rb ROLES = ['developer','contractor'] ROLES.each do |role| self.class_eval &lt;&lt;-eos has_many :#{role.downcase}s, :through =&gt; :project_users, :source =&gt; :user, :conditions =&gt; "project_users.role = '#{role}'" do def &lt;&lt;(user) proxy_owner.project_users.create(:role =&gt; '#{role}', :user =&gt; user) end end eos end </code></pre> <p>Looking back at everything above it doesn't seem like <em>the rails way</em> of doing things. Scoping this should make it possible to get the build and create commands working without redefining everything.</p> <p>Hope this helps!</p>
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    1. PORails 3 has_many :through + join table conditions / scoping
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    1. COThanks for your answer. It solves my problem, but doesn't do so in the way I was hoping - by using scopes on the `ProjectUsers` model. Declaring `:conditions => "project_users.role = '#{role}'"` doesn't seem very railsy, as I would have liked call something like `:conditions => { :scope => :developer }`. I'm still certain this is possible somehow. Either way, I'm awarding you the bounty for your effort, though this answer won't be marked as correct. Thanks for your input!
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    2. COThanks. I was really expecting `has_many :designers, :through => :project_users, :source => :user, :conditions => {:project_users => {:role => :designer}}` to work automatically, but apparently nested conditions hashes are not scoped with build and create. This was the best I could come up with.
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    3. COwhat is `proxy_owner` in the above? is that reference to the other side of the assiociation, similar to `proxy_association` that you get with `:extend`?
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