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    <p>You can tell if an association is loaded with <code>loaded?</code>. </p> <p>What is happening here, if I understand your problem, is that you are trying to run a finder on an ActiveRecord::Relation. Quickly browsing through <a href="https://github.com/rails/rails/blob/c9f27e3a479e17cd94248c007e4f528026bc18f6/activerecord/lib/active_record/relation/finder_methods.rb#L95" rel="nofollow">the code</a>, it does not appear that it will try to see if a collection is loaded before it issues the query. It does, however, take a block that will avoid multiple queries. For example (the model names have been changed because I am using a sample project I created for another question):</p> <pre><code>c = Canteen.first Canteen Load (0.2ms) SELECT "canteens".* FROM "canteens" LIMIT 1 =&gt; #&lt;Canteen id: 1, name: "Really good place", created_at: "2012-12-13 00:04:11", updated_at: "2012-12-13 00:04:11"&gt; c.meals.loaded? =&gt; false c.meals.find {|m| m.id == 3} Meal Load (0.2ms) SELECT "meals".* FROM "meals" WHERE "meals"."canteen_id" = 1 =&gt; #&lt;Meal id: 3, canteen_id: 1, name: "Banana Pie", price: #&lt;BigDecimal:7fcb6784fa78,'0.499E1',18(45)&gt;, created_at: "2012-12-13 00:37:41", updated_at: "2012-12-13 00:37:41"&gt; </code></pre> <p>You see in the last example that ActiveRecord issues the query to load the associated records. This is because ActiveRecord is calling <code>to_a</code> on the association, forcing the entire set to be loaded, and then filtering based on the block conditions. Obviously, this is not ideal.</p> <p>Let's try again, eager loading the association.</p> <pre><code>c = Canteen.includes(:meals).first Canteen Load (0.2ms) SELECT "canteens".* FROM "canteens" LIMIT 1 Meal Load (0.2ms) SELECT "meals".* FROM "meals" WHERE "meals"."canteen_id" IN (1) =&gt; #&lt;Canteen id: 1, name: "Really good place", created_at: "2012-12-13 00:04:11", updated_at: "2012-12-13 00:04:11"&gt; c.meals.loaded? =&gt; true c.meals.find {|m| m.id == 3} =&gt; #&lt;Meal id: 3, canteen_id: 1, name: "Banana Pie", price: #&lt;BigDecimal:7fcb68b596f0,'0.499E1',18(45)&gt;, created_at: "2012-12-13 00:37:41", updated_at: "2012-12-13 00:37:41"&gt; </code></pre> <p>In the last example here, you see that the collection is not loaded again. Instead, the block is used to filter the already loaded records.</p> <p>As you can see below, even if the records are loaded, ActiveRecord will issue a query to grab the associated record:</p> <pre><code>c.meals.loaded? =&gt; true c.meals.find(1) Meal Load (0.1ms) SELECT "meals".* FROM "meals" WHERE "meals"."canteen_id" = 1 AND "meals"."id" = ? LIMIT 1 [["id", 1]] =&gt; #&lt;Meal id: 1, canteen_id: 1, name: "Enchiladas", price: #&lt;BigDecimal:7fcb6584ce88,'0.699E1',18(45)&gt;, created_at: "2012-12-13 00:04:40", updated_at: "2012-12-13 00:04:40"&gt; SELECT "meals".* FROM "meals" WHERE "meals"."canteen_id" = 1 AND "meals"."id" = 3 =&gt; [#&lt;Meal id: 3, canteen_id: 1, name: "Banana Pie", price: #&lt;BigDecimal:7fcb68b808e0,'0.499E1',18(45)&gt;, created_at: "2012-12-13 00:37:41", updated_at: "2012-12-13 00:37:41"&gt;] </code></pre>
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