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copied!<p>In rails the <code>joins(:symbol)</code> statement turns into an inner join which is not suited for finding relations that don't exist. You can write the join by hand in order to make it a left join:</p> <pre><code>Cause.joins(""" LEFT JOIN cause_user_memberships ON cause_user_memberships.cause_id = causes.id AND cause_user_memberships.user_id = #{current_user.id} """).where("cause_user_memberships.id IS NULL") </code></pre> <p><strong>UPDATED</strong></p> <p>The inner join will prevent the creation of a set of Causes that don't have a mapping to the given user. For example:</p> <pre><code>Causes id | name ============ 1 | Cause 1 2 | Cause 2 Users id | name ============ 1 | User 1 Cause User Memberships id | user_id | cause_id ======================= 1 | 1 | 1 SELECT * FROM causes INNER JOIN cause_user_memberships ON cause_user_memberships.cause_id = causes.id AND cause_user_memberships.user_id = 1 </code></pre> <p>Will return</p> <pre><code>causes.id | causes.name | cause_user_memberships.id | cause_user_memberships.user_id | cause_user_memberships.cause_id =========================================================================================================================== 1 | Cause 1 | 1 | 1 | 1 </code></pre> <p>Which you can't perform any more logic on in order to tease out the causes the user doesn't belong to.</p> <pre><code>SELECT * FROM causes LEFT JOIN cause_user_memberships ON cause_user_memberships.cause_id = cause.id AND cause_user_memberships.user_id = 1 </code></pre> <p>Will return</p> <pre><code>causes.id | causes.name | cause_user_memberships.id | cause_user_memberships.user_id | cause_user_memberships.cause_id =========================================================================================================================== 1 | Cause 1 | 1 | 1 | 1 2 | Cause 2 | null | null | null </code></pre> <p>In this case all the Causes will get a row whether they match a record in the <code>cause_user_memberships</code> table or not. Now you can apply additional conditions to tease out the causes the user doesn't belong to (where cause_user_membership.id is null).</p>

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