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copied!<p>Optimistic locking is the way to go, but as you might have noticed already, your code will never raise ActiveRecord::StaleObjectError, since child object creation in has_many association skips the locking mechanism. Take a look at the following SQL:</p> <pre><code>UPDATE `scheduled_runs` SET `lock_version` = COALESCE(`lock_version`, 0) + 1, `attendances_count` = COALESCE(`attendances_count`, 0) + 1 WHERE (`id` = 113338481) </code></pre> <p>When you update attributes in the <strong>parent</strong> object, you usually see the following SQL instead:</p> <pre><code>UPDATE `scheduled_runs` SET `updated_at` = '2010-07-23 10:44:19', `lock_version` = 2 WHERE id = 113338481 AND `lock_version` = 1 </code></pre> <p>The above statement shows how optimistic locking is implemented: Notice the <code>lock_version = 1</code> in WHERE clause. When race condition happens, concurrent processes try to run this exact query, but only the first one succeeds, because the first one atomically updates the lock_version to 2, and subsequent processes will fail to <strong>find</strong> the record and raise ActiveRecord::StaleObjectError, since the same record doesn't have <code>lock_version = 1</code> any longer.</p> <p>So, in your case, a possible workaround is to touch the parent right before you create/destroy a child object, like so:</p> <pre><code>def attend(user) self.touch # Assuming you have updated_at column attendance = self.attendances.create(:user_id => user.id) rescue ActiveRecord::StaleObjectError #...do something... end </code></pre> <p>It's not meant to strictly avoid race conditions, but practically it should work in most cases.</p>

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