nodejs + mongodb: Modify existing document

Question

I store balances of users in mongo. Now I want to mathematically add to an existing balance as in balance = balance + amount. I am looping over multiple values in a function which gets an array of balances that are to be updated. The following

UserModel.findOneAndUpdate({ address: accounts[c].address }, 
  { balance: accounts[c].amount }, 
  (error, user) => { 
    // do stuff
  }
);

What I want is something like:

  { balance: user.amount + accounts[c].amount },

I also tried to first read the value and in the callback function do another findOneAndUpdate, unfortunately the callback handler does not have the right index c and thus accesses the wrong element in accounts[c].amount.

Just to make it clear, you want to update balance field with accounts[c].amount? — Emin Laletovic
– Emin Laletovic, Commented Feb 24, 2017 at 15:37
no, I want to add the amount. as in balance = balance + accounts[c].amount. NB: With add I mean the mathematical operation on numbers, not adding an element. — SCBuergel
– SCBuergel, Commented Feb 24, 2017 at 15:45

Emin Laletovic · Accepted Answer · 2017-02-24 15:50:38Z

1

Instead { balance: accounts[c].amount } try { $inc: { balance: accounts[c].amount }}. It should just increment balance by accounts[c].amount.

answered Feb 24, 2017 at 15:50

Emin Laletovic

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