I recently uncovered one of the most elusive bugs I've ever dealt with — and it had been quietly sitting in my codebase for over 3 years.<figure class="image"><img src="https://images.unsplash.com/photo-1523151594509-9d2e49774fec?crop=entropy&amp;cs=tinysrgb&amp;fit=max&amp;fm=jpg&amp;ixid=M3wxNTQwMTB8MHwxfHNlYXJjaHw3fHxidWd8ZW58MHx8fHwxNzQ4MjI0ODk5fDA&amp;ixlib=rb-4.1.0&amp;q=80&amp;w=1080"><figcaption>Photo by <a target="_blank" rel="noopener noreferrer" href="https://unsplash.com/@davidclode?utm_source=DNX&amp;utm_medium=referral">David Clode</a> on <a target="_blank" rel="noopener noreferrer" href="https://unsplash.com/?utm_source=DNX&amp;utm_medium=referral">Unsplash</a></figcaption></figure><h3>🔍 The Symptom</h3>Out of nowhere, MongoDB logs were showing slow queries against the wrong collections:<pre><code class="language-plaintext">ns: swimdb.strokes2324
query: { ageGroup: "13-14", ... }
</code></pre>But that didn’t make sense — this should have used the <code>best_strokes2324</code> collection.<h3>💡 The Suspicion</h3>My service uses a dynamic <code>setModel()</code> method to switch MongoDB collections based on query parameters. It looked fine:<pre><code class="language-plaintext">setModel = (collectionName) =&gt; {
 this.app.get("mongoClient").then((db) =&gt; {
 this.Model = db.collection(collectionName);
 });
};
</code></pre>And in the hook:<pre><code class="language-plaintext">await context.service.setModel("best_strokes2324");
</code></pre>Looks safe, right? 😅<h3>🧨 The Bug</h3>The <code>setModel()</code> function wasn’t returning anything — meaning that the <code>await</code> did absolutely nothing.Requests were racing ahead before the model was actually updated. As a result:Some queries hit the right collectionOthers hit a stale or incorrect oneAnd the worst part: it all looked fine most of the time<h3>🧠 The Fix</h3>One word: <code>async</code>.<pre><code class="language-plaintext">setModel = async (collectionName) =&gt; {
 const db = await this.app.get("mongoClient");
 this.Model = db.collection(collectionName);
};
</code></pre>Or if you still use <code>.then()</code>:<pre><code class="language-plaintext">setModel = (collectionName) =&gt;
 this.app.get("mongoClient").then((db) =&gt; {
 this.Model = db.collection(collectionName);
 });
</code></pre>Now <code>await setModel(...)</code> actually waits.<h3>🚦 What I Learned</h3>A missing <code>return</code> inside an async-like method can silently break everything.If your service state depends on timing, you must understand whether the mutation is blocking.MongoDB slow query logs are gold — they helped me trace the issue when nothing else would.<h3>🧠 Bonus: Why We Also Call <code>setModel()</code> in the <code>after</code> Hook</h3>You might wonder — if we already called <code>await setModel()</code> in the <code>before</code> hook, why call it again in the <code>after</code> hook?The answer is subtle but critical:<blockquote><code>context.service.Model</code> is shared across all requests. If another request comes in and sets a different model before our <code>after</code> hook runs, we could end up running queries against the wrong collection — even if we set it earlier.</blockquote>Feathers services are singletons. The model (<code>this.Model</code>) isn't scoped per request — it's global. But <code>context.params</code> is request-specific. So we store <code>season</code> in <code>context.params._season</code> during the <code>before</code> hook and re-call <code>setModel()</code> in the <code>after</code> hook, just before any late queries like <code>.distinct()</code>.That one extra line protects us from race conditions:<pre><code class="language-plaintext">await context.service.setModel("collection_prefix" + context.params._season);
</code></pre>This ensures the model is always correct at the time it’s needed — even if another user is hammering the server with overlapping requests. 🏁 ResultAfter the fix:No more inconsistent collectionsQuery times dropped dramaticallyConfidence restored

Just discovered a 3-year-old bug in my Feathers.js service: I was calling await setModel(...), but forgot to return a Promise inside setModel. The result? Queries sometimes hit the wrong MongoDB collection due to race conditions. It worked most of the time — until I checked the slow query logs. One missing return caused years of silent issues. Fixed now, and performance is back. Lesson learned.

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