fix(document): allow calling $model() with no args for TypeScript

Fix #13878

lib/model.js

@@ -1060,40 +1060,45 @@ Model.prototype.$__deleteOne = function $__deleteOne(options, cb) {
 };
 
 /**
- * Returns another Model instance.
+ * Returns the model instance used to create this document if no `name` specified.
+ * If `name` specified, returns the model with the given `name`.
  *
  * #### Example:
  *
- *     const doc = new Tank;
- *     await doc.model('User').findById(id);
+ *     const doc = new Tank({});
+ *     doc.$model() === Tank; // true
+ *     await doc.$model('User').findById(id);
  *
- * @param {String} name model name
- * @method model
+ * @param {String} [name] model name
+ * @method $model
  * @api public
  * @return {Model}
  */
 
-Model.prototype.model = function model(name) {
+Model.prototype.$model = function $model(name) {
+  if (arguments.length === 0) {
+    return this.constructor;
+  }
   return this[modelDbSymbol].model(name);
 };
 
 /**
- * Returns another Model instance.
+ * Returns the model instance used to create this document if no `name` specified.
+ * If `name` specified, returns the model with the given `name`.
  *
  * #### Example:
  *
- *     const doc = new Tank;
- *     await doc.model('User').findById(id);
+ *     const doc = new Tank({});
+ *     doc.$model() === Tank; // true
+ *     await doc.$model('User').findById(id);
  *
- * @param {String} name model name
- * @method $model
+ * @param {String} [name] model name
+ * @method model
  * @api public
  * @return {Model}
  */
 
-Model.prototype.$model = function $model(name) {
-  return this[modelDbSymbol].model(name);
-};
+Model.prototype.model = Model.prototype.$model;
 
 /**
  * Returns a document with `_id` only if at least one document exists in the database that matches

test/document.test.js

@@ -12521,6 +12521,14 @@ describe('document', function() {
     assert.strictEqual(attachmentSchemaPreValidateCalls, 1);
   });
 
+  it('returns constructor if using $model() with no args (gh-13878)', async function() {
+    const testSchema = new Schema({ name: String });
+    const Test = db.model('Test', testSchema);
+
+    const doc = new Test();
+    assert.strictEqual(doc.$model(), Test);
+  });
+
   it('avoids creating separate subpaths entry for every element in array (gh-13874)', async function() {
     const tradeSchema = new mongoose.Schema({ tradeId: Number, content: String });
 

test/types/document.test.ts

@@ -294,6 +294,16 @@ function gh12290() {
   user.isDirectModified('name');
 }
 
+function gh13878() {
+  const schema = new Schema({
+    name: String,
+    age: Number
+  });
+  const User = model('User', schema);
+  const user = new User({ name: 'John', age: 30 });
+  expectType<typeof User>(user.$model());
+}
+
 function gh13094() {
   type UserDocumentNever = HydratedDocument<{ name: string }, Record<string, never>>;
 

types/document.d.ts

@@ -75,6 +75,7 @@ declare module 'mongoose' {
 
     /** Returns the model with the given name on this document's associated connection. */
     $model<ModelType = Model<unknown>>(name: string): ModelType;
+    $model<ModelType = Model<DocType>>(): ModelType;
 
     /**
      * A string containing the current operation that Mongoose is executing