title: can't add an expression for a subfield in MongoDB
body: <p>Here is my code.</p>
  
  <pre><code>   Cart
          .aggregate()
          .match(where)
          .lookup({
              from: "product_catelogs",
              foreignField: "_id",
              localField: "product_id",
              as: "product"
          })
          .project({
            product: {
              $arrayElemAt: ["$product", 0]
            },
            "product.custom_field": {
              $add: [1, 1]
            },
            product_quantity: 1,
            product_variant: 1,
            guest_token: 1,
            user_id: 1,
            product_id: 1,
          })
  </code></pre>
  
  <p>In this I want to add a custom field with product object. but when I am trying to do so, I am facing this kind of error,</p>
  
  <blockquote>
    <p>can't add an expression for a subfield of product because there is
    already an expression that applies to the whole field</p>
  </blockquote>
  
  <p>Please help me out.</p>