add type def for Aggreaget#model without arguments

test/types/aggregate.test.ts

@@ -1,4 +1,4 @@
-import { Schema, model, Document, Expression, PipelineStage, Types } from 'mongoose';
+import { Schema, model, Document, Expression, PipelineStage, Types, Model, Aggregate } from 'mongoose';
 import { expectType } from 'tsd';
 
 const schema: Schema = new Schema({ name: { type: 'String' } });
@@ -8,6 +8,7 @@ interface ITest extends Document {
 }
 
 const Test = model<ITest>('Test', schema);
+const AnotherTest = model<ITest>('AnotherTest', schema);
 
 Test.aggregate([{ $match: { name: 'foo' } }]).exec().then((res: any) => console.log(res));
 
@@ -60,6 +61,10 @@ async function run() {
   expectType<ITest[]>(await Test.aggregate<ITest>().sort({ name: 'desc' }));
   expectType<ITest[]>(await Test.aggregate<ITest>().sort({ name: 'descending' }));
   expectType<ITest[]>(await Test.aggregate<ITest>().sort({ name: { $meta: 'textScore' } }));
+
+  // Aggregate.prototype.model()
+  expectType<Model<any>>(Test.aggregate<ITest>().model());
+  expectType<Aggregate<ITest[]>>(Test.aggregate<ITest>().model(AnotherTest));
 }
 
 function gh12017_1() {

types/aggregate.d.ts

@@ -166,6 +166,11 @@ declare module 'mongoose' {
      */
     model(model: Model<any>): this;
 
+    /**
+     * Returns the current model bound to this aggregate object
+     */
+    model(): Model<any>;
+
     /**
      * Append a new $near operator to this aggregation pipeline
      * @param arg $near operator contents