fix(document): allow manually populating path within document array

Fix #8273

Author: vkarpov15

lib/document.js

@@ -1167,6 +1167,20 @@ Document.prototype.$set = function $set(path, val, type, options) {
       val = schema.applySetters(val, this, false, priorVal);
     }
 
+    if (schema.$isMongooseDocumentArray &&
+        Array.isArray(val) &&
+        val.length > 0 &&
+        val[0].$__ != null &&
+        val[0].$__.populated != null) {
+      const populatedPaths = Object.keys(val[0].$__.populated);
+      for (const populatedPath of populatedPaths) {
+        this.populated(path + '.' + populatedPath,
+          val.map(v => v.populated(populatedPath)),
+          val[0].$__.populated[populatedPath].options);
+      }
+      didPopulate = true;
+    }
+
     if (!didPopulate && this.$__.populated) {
       delete this.$__.populated[path];
     }

lib/schematype.js

@@ -1265,7 +1265,7 @@ SchemaType._isRef = function(self, value, doc, init) {
     // - setting / pushing values after population
     const path = doc.$__fullPath(self.path);
     const owner = doc.ownerDocument ? doc.ownerDocument() : doc;
-    ref = owner.populated(path);
+    ref = owner.populated(path) || doc.populated(self.path);
   }
 
   if (ref) {

lib/types/documentarray.js

@@ -86,7 +86,7 @@ class CoreDocumentArray extends CoreMongooseArray {
         }
       }
     }
-
+    
     if (Constructor.$isMongooseDocumentArray) {
       return Constructor.cast(value, this, undefined, undefined, index);
     }

test/model.discriminator.test.js

@@ -1508,5 +1508,37 @@ describe('model', function() {
       });
       assert.strictEqual(doc.operations[0].pitchPath.path[0]._id, void 0);
     });
+
+    it('with discriminators in embedded arrays (gh-8273)', function(done) {
+      const ProductSchema = new Schema({
+        title: String
+      });
+      const Product = mongoose.model('gh8273_Product', ProductSchema);
+      const ProductItemSchema = new Schema({
+        product: { type: Schema.Types.ObjectId, ref: 'gh8273_Product' }
+      });
+
+      const OrderItemSchema = new Schema({}, {discriminatorKey: '__t'});
+
+      const OrderSchema = new Schema({
+        items: [OrderItemSchema],
+      });
+
+      OrderSchema.path('items').discriminator('ProductItem', ProductItemSchema);
+      const Order = mongoose.model('Order', OrderSchema);
+
+      const product = new Product({title: 'Product title'});
+
+      const order = new Order({
+        items: [{
+          __t: 'ProductItem',
+          product: product
+        }]
+      });
+      assert.ok(order.items[0].product.title);
+      assert.equal(order.populated('items.product').length, 1);
+
+      done();
+    });
   });
 });