fix(populate): correctly set populatedModelSymbol on documents populated using Model.populate()

lib/helpers/populate/getModelsMapForPopulate.js

@@ -540,6 +540,7 @@ function addModelNamesToMap(model, map, available, modelNames, options, data, re
       // Used internally for checking what model was used to populate this
       // path.
       options[populateModelSymbol] = Model;
+      currentOptions[populateModelSymbol] = Model;
       available[modelName] = {
         model: Model,
         options: currentOptions,

lib/model.js

@@ -4264,7 +4264,7 @@ const excludeIdReg = /\s?-_id\s?/;
 const excludeIdRegGlobal = /\s?-_id\s?/g;
 
 function populate(model, docs, options, callback) {
-  const populateOptions = { ...options };
+  const populateOptions = options;
   if (options.strictPopulate == null) {
     if (options._localModel != null && options._localModel.schema._userProvidedOptions.strictPopulate != null) {
       populateOptions.strictPopulate = options._localModel.schema._userProvidedOptions.strictPopulate;

lib/types/array/methods/index.js

@@ -226,6 +226,9 @@ const methods = {
     if (populated && value !== null && value !== undefined) {
       // cast to the populated Models schema
       Model = populated.options[populateModelSymbol];
+      if (Model == null) {
+        throw new MongooseError('No populated model found for path `' + this[arrayPathSymbol] + '`. This is likely a bug in Mongoose, please report an issue on github.com/Automattic/mongoose.');
+      }
 
       // only objects are permitted so we can safely assume that
       // non-objects are to be interpreted as _id

test/model.populate.test.js

@@ -10504,4 +10504,39 @@ describe('model: populate:', function() {
     });
     assert.deepStrictEqual(doc.children.map(c => c.name).sort(), ['Chad', 'Luke']);
   });
+
+  it('allows pushing to model populated in a query cursor (gh-13575)', async function() {
+    const Company = db.model('Company', Schema({ name: String }));
+    const User = db.model(
+      'User',
+      Schema({ name: String, companies: [{ type: Schema.Types.ObjectId, ref: 'Company' }] })
+    );
+
+    await User.deleteMany({});
+    await Company.insertMany([{ name: 'Company 1' }, { name: 'Company 2' }]);
+    const company = await Company.findOne({ name: 'Company 1' });
+    const company2 = await Company.findOne({ name: 'Company 2' });
+
+    await User.insertMany([{ name: 'User 1', companies: [company.id] }]);
+
+    await User.find()
+      .populate('companies')
+      .cursor()
+      .eachAsync(async(user) => {
+        if (!user.populated('companies')) {
+          await user.populate('companies');
+        }
+
+        user.companies.push(company2);
+
+        await user.save();
+      });
+
+    const user = await User.findOne();
+    assert.equal(user.name, 'User 1');
+    assert.deepEqual(
+      user.toObject().companies.sort().map(v => v.toString()),
+      [company._id.toString(), company2._id.toString()]
+    );
+  });
 });