Avoid depopulating populated subdocs underneath document arrays when copying to another document

lib/document.js

@@ -1087,7 +1087,11 @@ Document.prototype.$set = function $set(path, val, type, options) {
       if (path.$__isNested) {
         path = path.toObject();
       } else {
-        path = path._doc;
+        // This ternary is to support gh-7898 (copying virtuals if same schema)
+        // while not breaking gh-10819, which for some reason breaks if we use toObject()
+        path = path.$__schema === this.$__schema
+          ? applyVirtuals(path, { ...path._doc })
+          : path._doc;
       }
     }
     if (path == null) {
@@ -4012,11 +4016,11 @@ function applyVirtuals(self, json, options, toObjectOptions) {
     ? toObjectOptions.aliases
     : true;
 
+  options = options || {};
   let virtualsToApply = null;
   if (Array.isArray(options.virtuals)) {
     virtualsToApply = new Set(options.virtuals);
-  }
-  else if (options.virtuals && options.virtuals.pathsToSkip) {
+  } else if (options.virtuals && options.virtuals.pathsToSkip) {
     virtualsToApply = new Set(paths);
     for (let i = 0; i < options.virtuals.pathsToSkip.length; i++) {
       if (virtualsToApply.has(options.virtuals.pathsToSkip[i])) {
@@ -4029,7 +4033,6 @@ function applyVirtuals(self, json, options, toObjectOptions) {
     return json;
   }
 
-  options = options || {};
   for (i = 0; i < numPaths; ++i) {
     path = paths[i];
 

lib/model.js

@@ -5124,7 +5124,7 @@ function _assign(model, vals, mod, assignmentOpts) {
       }
       // flag each as result of population
       if (!lean) {
-        val.$__.wasPopulated = val.$__.wasPopulated || true;
+        val.$__.wasPopulated = val.$__.wasPopulated || { value: _val };
       }
     }
   }

lib/schema/documentarray.js

@@ -443,19 +443,9 @@ DocumentArrayPath.prototype.cast = function(value, doc, init, prev, options) {
 
     const Constructor = getConstructor(this.casterConstructor, rawArray[i]);
 
-    // Check if the document has a different schema (re gh-3701)
-    if (rawArray[i].$__ != null && !(rawArray[i] instanceof Constructor)) {
-      const spreadDoc = handleSpreadDoc(rawArray[i], true);
-      if (rawArray[i] !== spreadDoc) {
-        rawArray[i] = spreadDoc;
-      } else {
-        rawArray[i] = rawArray[i].toObject({
-          transform: false,
-          // Special case: if different model, but same schema, apply virtuals
-          // re: gh-7898
-          virtuals: rawArray[i].schema === Constructor.schema
-        });
-      }
+    const spreadDoc = handleSpreadDoc(rawArray[i], true);
+    if (rawArray[i] !== spreadDoc) {
+      rawArray[i] = spreadDoc;
     }
 
     if (rawArray[i] instanceof Subdocument) {

lib/schematype.js

@@ -1505,7 +1505,7 @@ SchemaType.prototype._castRef = function _castRef(value, doc, init) {
   }
 
   if (value.$__ != null) {
-    value.$__.wasPopulated = value.$__.wasPopulated || true;
+    value.$__.wasPopulated = value.$__.wasPopulated || { value: value._id };
     return value;
   }
 
@@ -1531,7 +1531,7 @@ SchemaType.prototype._castRef = function _castRef(value, doc, init) {
     !doc.$__.populated[path].options.options ||
     !doc.$__.populated[path].options.options.lean) {
     ret = new pop.options[populateModelSymbol](value);
-    ret.$__.wasPopulated = true;
+    ret.$__.wasPopulated = { value: ret._id };
   }
 
   return ret;

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() {