feat(query): add overwriteDiscriminatorKey option that allows changin…

…g the discriminator key in `findOneAndUpdate()`, `updateOne()`, etc.

Fix #6087

lib/query.js

@@ -1293,7 +1293,8 @@ Query.prototype.getOptions = function() {
  * - [writeConcern](https://docs.mongodb.com/manual/reference/method/db.collection.update/)
  * - [timestamps](https://mongoosejs.com/docs/guide.html#timestamps): If `timestamps` is set in the schema, set this option to `false` to skip timestamps for that particular update. Has no effect if `timestamps` is not enabled in the schema options.
  * - omitUndefined: delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
- *
+ * - overwriteDiscriminatorKey: allow setting the discriminator key in the update. Will use the correct discriminator schema if the update changes the discriminator key.
+ * 
  * The following options are only for `find()`, `findOne()`, `findById()`, `findOneAndUpdate()`, and `findByIdAndUpdate()`:
  *
  * - [lean](./api.html#query_Query-lean)
@@ -1361,6 +1362,10 @@ Query.prototype.setOptions = function(options, overwrite) {
     this._mongooseOptions.setDefaultsOnInsert = options.setDefaultsOnInsert;
     delete options.setDefaultsOnInsert;
   }
+  if ('overwriteDiscriminatorKey' in options) {
+    this._mongooseOptions.overwriteDiscriminatorKey = options.overwriteDiscriminatorKey;
+    delete options.overwriteDiscriminatorKey;
+  }
 
   return Query.base.setOptions.call(this, options);
 };
@@ -4485,6 +4490,19 @@ Query.prototype._post = function(fn) {
 
 Query.prototype._castUpdate = function _castUpdate(obj, overwrite) {
   let strict;
+  let schema = this.schema;
+
+  const discriminatorKey = schema.options.discriminatorKey;
+  const baseSchema = schema._baseSchema ? schema._baseSchema : schema;
+  if (this._mongooseOptions.overwriteDiscriminatorKey &&
+      obj[discriminatorKey] != null &&
+      baseSchema.discriminators) {
+    const _schema = baseSchema.discriminators[obj[discriminatorKey]];
+    if (_schema != null) {
+      schema = _schema;
+    }
+  }
+
   if ('strict' in this._mongooseOptions) {
     strict = this._mongooseOptions.strict;
   } else if (this.schema && this.schema.options) {
@@ -4508,7 +4526,6 @@ Query.prototype._castUpdate = function _castUpdate(obj, overwrite) {
     upsert = this.options.upsert;
   }
 
-  let schema = this.schema;
   const filter = this._conditions;
   if (schema != null &&
       utils.hasUserDefinedProperty(filter, schema.options.discriminatorKey) &&

test/model.update.test.js

@@ -3478,4 +3478,33 @@ describe('model: updateOne: ', function() {
       });
     });
   });
+
+  describe('overwriteDiscriminatorKey', function() {
+    it('allows changing discriminator key in update (gh-6087)', function() {
+      const baseSchema = new Schema({}, { discriminatorKey: 'type' });
+      const baseModel = db.model('Test', baseSchema);
+
+      const aSchema = Schema({ aThing: Number }, { _id: false, id: false });
+      const aModel = baseModel.discriminator('A', aSchema);
+
+      const bSchema = new Schema({ bThing: String }, { _id: false, id: false });
+      const bModel = baseModel.discriminator('B', bSchema);
+
+      return co(function*() {
+        // Model is created as a type A
+        let doc = yield baseModel.create({ type: 'A', aThing: 1 });
+
+        yield aModel.updateOne(
+          { _id: doc._id },
+          { type: 'B', bThing: 'two' },
+          { runValidators: true, overwriteDiscriminatorKey: true }
+        );
+
+        doc = yield baseModel.findById(doc);
+        assert.equal(doc.type, 'B');
+        assert.ok(doc instanceof bModel);
+        assert.equal(doc.bThing, 'two');
+      });
+    });
+  });
 });