Update transactional value in mongodb

Question

i am making a mern project wehre i need to update two fields in mongo db record .

i have two fields one is array of object named as Wallets and another is transaction.

i am getting a object from frontend which include the amount,index of wallet from which the transaction is made as wallets is array of object in mongo.

now i need to subtract the amount of transaction from the wallet. how to do it i have written a controller in node.js but its not doing anything.

my object that i receive from front end if..

transaction = { amount: '50', wallet: 0, type: 'expense', description: 'kmop' };

my node js controller is...

module.exports.addTransaction = async (req, res) => {
  const transaction = req.body;
  const {wallet} = transaction;

  try {
      const walletName = `Wallets.${wallet}.amount`
      await UserModel.findOneAndUpdate(
        { email: email },
        {  $inc: { walletName : -transaction.amount}   ,
          $push: { transactions: { $each: [transaction], $position: 0 } } }
      );
      res.send({stat: true});
    } 
    else {
      console.log(data)
      res.send({ stat: false });
    }
  } catch (err) {
    console.log("err",err)
    res.send({ stat: false, err: err.message });
  }
};

my mongo db record--

{"_id":{"$oid":"64a5a396ec152ab4f5e1c60c"},
"firstName":"Vinayak",
"lastName":"Pandey",
"email":"[email protected]",
"password":"$2b$10$dpAmLPa9imDKn3qeyhZ/hOwolO6NOYVwNvVgBc9PY8XZZ3n4WYh/O",
"Wallets":[{"name":"paytm","amount":500},
           {"name":"hdfc","amount":5000},
           {"name":"test","amount":3020}]

Suppose i made a transaction from wallet namde paytm of rs 50 so i want that is the amount in object with name as paytm should be decreased by rs-50 so initially it was rs500 after the process it should became rs450....

Answer 1

To apply $inc operator at an element of an array. We should use dot notation to specify a <field> in an embedded document or in an array,

The positional $

The $ operator identifies an element in an array to update without explicitly specifying the position of the element in the array.

db.collection.update({
  email: "[email protected]",
  "Wallets.name": "paytm"
},
{
  "$inc": {
    "Wallets.$.amount": -50
  }
})

mongoplayground

Input:

[
  {
    "_id": "64a5a396ec152ab4f5e1c60c",
    "firstName": "Vinayak",
    "lastName": "Pandey",
    "email": "[email protected]",
    "password": "$2b$10$dpAmLPa9imDKn3qeyhZ/hOwolO6NOYVwNvVgBc9PY8XZZ3n4WYh/O",
    "Wallets": [
      {
        "name": "paytm",
        "amount": 500
      },
      {
        "name": "hdfc",
        "amount": 5000
      },
      {
        "name": "test",
        "amount": 3020
      }
    ]
  }
]

Output:

[
  {
    "Wallets": [
      {
        "amount": 450,
        "name": "paytm"
      },
      {
        "amount": 5000,
        "name": "hdfc"
      },
      {
        "amount": 3020,
        "name": "test"
      }
    ],
    "_id": "64a5a396ec152ab4f5e1c60c",
    "email": "[email protected]",
    "firstName": "Vinayak",
    "lastName": "Pandey",
    "password": "$2b$10$dpAmLPa9imDKn3qeyhZ/hOwolO6NOYVwNvVgBc9PY8XZZ3n4WYh/O"
  }
]

---

UPDATE

Update an element of array with a specific array index:

const idx = 1;
db.users.updateOne(
    {
        email: '[email protected]'
    },
    {
        $inc: {
            [`Wallets.${idx}.amount`]: -50,
        },
    },
);