My api code retrieves an empty data array from mongodb while the code works well on mongodb playground

Question

My mongodb database includes a collection of users and each user has an array of shops and each shop has an array of products. Here is a simplified version of my collection structure:

[
    {
        "_id": "60e66b70da2439232e330415",
        "name": "User1 Name",
        "shops": [
            {
                "_id": "60e7c9e2be0d8f03544a03b8",
                "shopName": "User1 Shop1 Name",
                "products": [
                    {
                        "_id": "60e9e9e8105d6021a2e91535",
                        "title": "User1 Shop1 Product1 Title"
                    },
                    {
                        "_id": "60e9f4a0105d6021a2e91536",
                        "title": "User1 Shop1 Product2 Title"
                    }
                ]
            },
            {
                "_id": "60e8e8c00f3986577cb968c9",
                "shopName": "User1 Shop2 Name",
                "products": [
                    {
                        "_id": "60e9f4fe105d6021a2e91537",
                        "title": "User1 Shop2 Product1 Title"
                    },
                    {
                        "_id": "60e9f515105d6021a2e91538",
                        "title": "User1 Shop2 Product2 Title"
                    }
                ]
            }
        ]
    },
    {
        "_id": "60e66b93da2439232e330416",
        "name": "User2 Name",
        "shops": [
            {
                "_id": "60e69698e76cad44e49e1fc8",
                "shopName": "User2 Shop1 Name",
                "products": [
                    {
                        "_id": "60e9f588105d6021a2e91539",
                        "title": "User2 Shop1 Product1 Title"
                    },
                    {
                        "_id": "60e9f59c105d6021a2e9153a",
                        "title": "User2 Shop1 Product2 Title"
                    }
                ]
            },
            {
                "_id": "60e902b441e9df63c7fbcb49",
                "shopName": "User2 Shop2 Name",
                "products": [
                    {
                        "_id": "60e9f5c9105d6021a2e9153b",
                        "title": "User2 Shop2 Product1 Title"
                    },
                    {
                        "_id": "60e9f5de105d6021a2e9153c",
                        "title": "User2 Shop2 Product2 Title"
                    }
                ]
            }
        ]
    }
]

I have an api endpoint like .../api/products/60e9f5de105d6021a2e9153c. This endpoint includes a parameter which is a productId. I have the following two codes to retrieve the product data from my mongodb collection, but the retrieved array is empty.

My endpoint code:

app.get("/api/products/:productId", (req,res)=>{
    let productId = req.params.productId;
    myData.getProductByProductId(productId)
    .then(products => res.json(products))
    .catch(err => res.json({"message": err}));
});

My DataService code:

getProductByProductId: function (productId) {
            return new Promise((resolve, reject) => {
                User.aggregate([
                    {
                      $match: {
                        "shops.products._id": productId
                      }
                    },
                    {
                      "$unwind": "$shops"
                    },
                    {
                      "$unwind": "$shops.products"
                    },
                    {
                      $match: {
                        "shops.products._id": productId
                      }
                    },
                    {
                      $project: {
                        "_id": "$shops.products._id",
                        "title": "$shops.products.title"
                      }
                    }
                  ])
                .exec().then(products => {
                    resolve(products)
                }).catch(err => {
                    reject(err);
                });
            });
        }

The aggregate code works well in this playground but in my web app code retrieves an empty array.

Please try User.aggregate(..).then(successCallback); Remove the exec() — Rajdeep D
– Rajdeep D, Commented Jul 21, 2021 at 17:01
Thanks @RajdeepDebnath. It gives me this output now: { "message": {} } — Omid
– Omid, Commented Jul 21, 2021 at 17:10
ok, that means error, you can try console logging then(products => { console.log(products) }).catch(err => { console.log(err); }) — Rajdeep D
– Rajdeep D, Commented Jul 21, 2021 at 17:12

Rajdeep D · Accepted Answer · 2021-07-21 18:28:45Z

1

Turns out the casting of the ObjectId seemed to be the issue. We need to use mongoose.Types.ObjectId

The API code will looks like this

getProductByProductId: function (productId) {
return new Promise((resolve, reject) => {
  User.aggregate([
  {
  $match: {
    "shops.products._id": mongoose.Types.ObjectId(productId)
  }
  },
  {
    "$unwind": "$shops"
  },
  {
    "$unwind": "$shops.products"
  },
  {
    $match: {
    "shops.products._id": mongoose.Types.ObjectId(productId)
  }
  },
  {
  $project: {
  "_id": "$shops.products._id",
  "title": "$shops.products.title"
  }
}
])
.then(products => {
resolve(products)
}).catch(err => {
reject(err);
});
});
}

answered Jul 21, 2021 at 18:28

Rajdeep D

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