docs(discriminators): add section about changing discriminator key

Fix #6087

Author: vkarpov15

docs/discriminators.md

@@ -31,12 +31,25 @@ instances.
 
 ### Discriminator keys
 
-The way mongoose tells the difference between the different
-discriminator models is by the 'discriminator key', which is
-`__t` by default. Mongoose adds a String path called `__t`
-to your schemas that it uses to track which discriminator
-this document is an instance of.
+The way Mongoose tells the difference between the different discriminator models is by the 'discriminator key', which is `__t` by default.
+Mongoose adds a String path called `__t` to your schemas that it uses to track which discriminator this document is an instance of.
 
 ```javascript
 [require:Discriminator keys]
 ```
+
+### Updating the discriminator key
+
+By default, Mongoose doesn't let you update the discriminator key.
+`save()` will throw an error if you attempt to update the discriminator key.
+And `findOneAndUpdate()`, `updateOne()`, etc. will strip out discriminator key updates.
+
+```javascript
+[require:Update discriminator key]
+```
+
+To update a document's discriminator key, use `findOneAndUpdate()` or `updateOne()` with the `overwriteDiscriminatorKey` option set as follows.
+
+```javascript
+[require:use overwriteDiscriminatorKey to change discriminator key]
+```

test/docs/discriminators.test.js

@@ -100,6 +100,42 @@ describe('discriminator docs', function() {
     assert.equal(event3.__t, 'SignedUp');
   });
 
+  it('Update discriminator key', async function() {
+    let event = new ClickedLinkEvent({ time: Date.now(), url: 'google.com' });
+    await event.save();
+
+    event.__t = 'SignedUp';
+    // ValidationError: ClickedLink validation failed: __t: Cast to String failed for value "SignedUp" (type string) at path "__t"
+    // acquit:ignore:start
+    await assert.rejects(async () => {
+    // acquit:ignore:end
+    await event.save();
+    // acquit:ignore:start
+    }, /__t: Cast to String failed/);
+    // acquit:ignore:end
+
+    event = await ClickedLinkEvent.findByIdAndUpdate(event._id, { __t: 'SignedUp' }, { new: true });
+    event.__t; // 'ClickedLink', update was a no-op
+    // acquit:ignore:start
+    assert.equal(event.__t, 'ClickedLink');
+    // acquit:ignore:end
+  });
+
+  it('use overwriteDiscriminatorKey to change discriminator key', async function() {
+    let event = new ClickedLinkEvent({ time: Date.now(), url: 'google.com' });
+    await event.save();
+
+    event = await ClickedLinkEvent.findByIdAndUpdate(
+      event._id,
+      { __t: 'SignedUp' },
+      { overwriteDiscriminatorKey: true, new: true }
+    );
+    event.__t; // 'SignedUp', updated discriminator key
+    // acquit:ignore:start
+    assert.equal(event.__t, 'SignedUp');
+    // acquit:ignore:end
+  });
+
   /**
    * Discriminator models are special; they attach the discriminator key
    * to queries. In other words, `find()`, `count()`, `aggregate()`, etc.