test(document): add test case for #14418

Author: vkarpov15

test/document.test.js

@@ -12476,6 +12476,68 @@ describe('document', function() {
     doc.set({ nested: void 0 });
     assert.strictEqual(doc.toObject().nested, void 0);
   });
+
+  it('avoids depopulating populated subdocs underneath document arrays when copying to another document (gh-14418)', async function() {
+    const cartSchema = new mongoose.Schema({
+      products: [
+        {
+          product: {
+            type: mongoose.Schema.Types.ObjectId,
+            ref: 'Product'
+          },
+          quantity: Number
+        }
+      ],
+      singleProduct: {
+        type: mongoose.Schema.Types.ObjectId,
+        ref: 'Product'
+      }
+    });
+    const purchaseSchema = new mongoose.Schema({
+      products: [
+        {
+          product: {
+            type: mongoose.Schema.Types.ObjectId,
+            ref: 'Product'
+          },
+          quantity: Number
+        }
+      ],
+      singleProduct: {
+        type: mongoose.Schema.Types.ObjectId,
+        ref: 'Product'
+      }
+    });
+    const productSchema = new mongoose.Schema({
+      name: String
+    });
+
+    const Cart = db.model('Cart', cartSchema);
+    const Purchase = db.model('Purchase', purchaseSchema);
+    const Product = db.model('Product', productSchema);
+
+    const dbProduct = await Product.create({ name: 'Bug' });
+
+    const dbCart = await Cart.create({
+      products: [
+        {
+          product: dbProduct,
+          quantity: 2
+        }
+      ],
+      singleProduct: dbProduct
+    });
+
+    const foundCart = await Cart.findById(dbCart._id).
+      populate('products.product singleProduct');
+
+    const purchaseFromDbCart = new Purchase({
+      products: foundCart.products,
+      singleProduct: foundCart.singleProduct
+    });
+    assert.equal(purchaseFromDbCart.products[0].product.name, 'Bug');
+    assert.equal(purchaseFromDbCart.singleProduct.name, 'Bug');
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is availabe', function() {