title: Typegoose class defined static method returns empty array
body: <p>When I use findByUserId defined static method, I get empty array value whereas if I use the model directly it will retrieve successfully  the object.</p>
  <pre><code>    export default class Cart {
      
          @prop({required: true, unique: true})
          public userId!: String
      
          @prop()
          public items?: [Item]
      
          public static async findByUserId(this: ReturnModelType&lt;typeof Cart&gt;, userId: string) {
              try {
                  return await this.find({itemId: userId});
              } catch (err) {
                  return null;
              }
          }
      }
  
      export const CartModel = getModelForClass(Cart);
      
      // RETURNS EMPTY ARRAY
       const user = await CartModel.findByUserId(&quot;asdasdsa21321&quot;)
  
     // WORKS
       const user1 = await CartModel.find({userId:&quot;asdasdsa21321&quot;})
  </code></pre>
  <p>Any ideas?</p>