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PORails idiom to avoid duplicates in has_many :through
 

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    <p>I have a standard many-to-many relationship between users and roles in my Rails app:</p> <pre><code>class User &lt; ActiveRecord::Base has_many :user_roles has_many :roles, :through =&gt; :user_roles end </code></pre> <p>I want to make sure that a user can only be assigned any role once. Any attempt to insert a duplicate should ignore the request, not throw an error or cause validation failure. What I really want to represent is a "set", where inserting an element that already exists in the set has no effect. {1,2,3} U {1} = {1,2,3}, not {1,1,2,3}.</p> <p>I realize that I can do it like this:</p> <pre><code>user.roles &lt;&lt; role unless user.roles.include?(role) </code></pre> <p>or by creating a wrapper method (e.g. <code>add_to_roles(role)</code>), but I was hoping for some idiomatic way to make it automatic via the association, so that I can write:</p> <pre><code>user.roles &lt;&lt; role # automatically checks roles.include? </code></pre> <p>and it just does the work for me. This way, I don't have to remember to check for dups or to use the custom method. Is there something in the framework I'm missing? I first thought the :uniq option to has_many would do it, but it's basically just "select distinct."</p> <p>Is there a way to do this declaratively? If not, maybe by using an association extension?</p> <p>Here's an example of how the default behavior fails:</p> <pre> &gt;&gt; <strong>u = User.create</strong> User Create (0.6ms) INSERT INTO "users" ("name") VALUES(NULL) =&gt; #&lt;User id: 3, name: nil&gt; &gt;&gt; <strong>u.roles &lt;&lt; Role.first</strong> Role Load (0.5ms) SELECT * FROM "roles" LIMIT 1 UserRole Create (0.5ms) INSERT INTO "user_roles" ("role_id", "user_id") VALUES(1, 3) Role Load (0.4ms) SELECT "roles".* FROM "roles" INNER JOIN "user_roles" ON "roles".id = "user_roles".role_id WHERE (("user_roles".user_id = 3)) =&gt; [#&lt;Role id: 1, name: "1"&gt;] &gt;&gt; <strong>u.roles &lt;&lt; Role.first</strong> Role Load (0.4ms) SELECT * FROM "roles" LIMIT 1 UserRole Create (0.5ms) INSERT INTO "user_roles" ("role_id", "user_id") VALUES(1, 3) <strong><em>=&gt; [#&lt;Role id: 1, name: "1"&gt;, #&lt;Role id: 1, name: "1"&gt;]</em></strong></pre>
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