docs: add Lodash guide highlighting issues with cloneDeep()

docs/guides.md

@@ -33,6 +33,7 @@ integrating Mongoose with external tools and frameworks.
 ### Integrations
 
 * [Promises](/docs/promises.html)
+* [Lodash](/docs/lodash.html)
 * [AWS Lambda](/docs/lambda.html)
 * [Browser Library](/docs/browser.html)
 * [GeoJSON](/docs/geojson.html)

docs/lodash.md

@@ -0,0 +1,39 @@
+# Using Mongoose with Lodash
+
+For the most part, Mongoose works well with [Lodash](https://lodash.com/).
+However, there are a few caveats that you should know about.
+
+* [`cloneDeep()`](#clonedeep)
+
+## `cloneDeep()`
+
+You should not use [Lodash's `cloneDeep()` function](https://lodash.com/docs/4.17.15#cloneDeep) on any Mongoose objects.
+This includes [connections](./connections.html), [model classes](./models.html), and [queries](./queries.html), but is _especially_ important for [documents](./documents.html).
+For example, you may be tempted to do the following:
+
+```javascript
+const _ = require('lodash');
+
+const doc = await MyModel.findOne();
+
+const newDoc = _.cloneDeep(doc);
+newDoc.myProperty = 'test';
+await newDoc.save();
+```
+
+However, the above code will throw the following error if `MyModel` has any array properties.
+
+```no-highlight
+TypeError: this.__parentArray.$path is not a function
+```
+
+This is because Lodash's `cloneDeep()` function doesn't [handle proxies](https://stackoverflow.com/questions/50663784/lodash-clonedeep-remove-proxy-from-object), and [Mongoose arrays are proxies as of Mongoose 6](https://thecodebarbarian.com/introducing-mongoose-6.html#arrays-as-proxies).
+You typically don't have to deep clone Mongoose documents, but, if you have to, use the following alternative to `cloneDeep()`:
+
+```javascript
+const doc = await MyModel.findOne();
+
+const newDoc = new MyModel().init(doc.toObject());
+newDoc.myProperty = 'test';
+await newDoc.save();
+```

docs/source/index.js

@@ -62,6 +62,7 @@ exports['docs/jobs.pug'] = {
   jobs
 };
 exports['docs/change-streams.md'] = { title: 'MongoDB Change Streams in NodeJS with Mongoose', markdown: true };
+exports['docs/lodash.md'] = { title: 'Using Mongoose with Lodash', markdown: true };
 
 for (const props of Object.values(exports)) {
   props.jobs = jobs;