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    <p>Here is the SQL that ActiveRecord generates for <code>user.cities</code>:</p> <pre><code>SELECT `cities`.* FROM `cities` INNER JOIN city_permissions ON (cities.id = city_permissions.city_id) WHERE (city_permissions.user_id = 1 ) </code></pre> <p>EXPLAIN results below:</p> <pre><code>+----+-------------+------------------+--------+---------------------------------------------------------------------+-----------------------------------+---------+-------------------------------------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------------+--------+---------------------------------------------------------------------+-----------------------------------+---------+-------------------------------------------------+------+-------------+ | 1 | SIMPLE | city_permissions | ref | index_city_permissions_on_user_id,index_city_permissions_on_city_id | index_city_permissions_on_user_id | 5 | const | 1 | Using where | | 1 | SIMPLE | cities | eq_ref | PRIMARY | PRIMARY | 4 | barhopolis_development.city_permissions.city_id | 1 | | +----+-------------+------------------+--------+---------------------------------------------------------------------+-----------------------------------+---------+-------------------------------------------------+------+-------------+ </code></pre> <p>And here's the SQL that ActiveRecord generates for <code>user.city_permissions</code>:</p> <pre><code>SELECT * FROM `city_permissions` WHERE (`city_permissions`.user_id = 1) </code></pre> <p>With the EXPLAIN results for that query:</p> <pre><code>+----+-------------+------------------+------+-----------------------------------+-----------------------------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------------+------+-----------------------------------+-----------------------------------+---------+-------+------+-------------+ | 1 | SIMPLE | city_permissions | ref | index_city_permissions_on_user_id | index_city_permissions_on_user_id | 5 | const | 1 | Using where | +----+-------------+------------------+------+-----------------------------------+-----------------------------------+---------+-------+------+-------------+ </code></pre> <p>Looks like it <strong>is indeed working correctly</strong>. From the MySQL Manual:</p> <h2>eq_ref</h2> <p>One row is read from this table for each combination of rows from the previous tables. Other than the system and const types, this is the best possible join type. It is used when all parts of an index are used by the join and the index is a PRIMARY KEY or UNIQUE index.</p> <h2>ref</h2> <p>All rows with matching index values are read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this is a good join type.</p>
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