How to use a variable as a field name in mongodb-native findAndModify?

Question

In this code that uses the mongodb-native driver I'd like to increase the value of the field which I specify in a separate variable. The problem is that the field name in the $inc clause will be 'variable' in this case, not the contents of the variable. In the query part the variable selected works as expected and finds the correct id.

var selected = 'id_of_the_selected_one';
var variable = 'some_string';
collection.findAndModify(
     {_id : selected}, 
     {},
     {$inc : {variable : 1}},
     {new : true, upsert : true},
     function(err, autoincrement) { /* ... */ }
);

How should I do it so that instead of the word 'variable' there will be the contents of the variable?

Answer 1

Set the key of another variable to its value and pass that as an object. Note of action:

var selected = 'id_of_the_selected_one';
var variable = 'some_string';
var action = {};
action[variable] = 1; // the value

collection.findAndModify(
    {_id : selected}, 
    {}, 
    {$inc : action}, 
    {new : true, upsert : true}, 
    function(err, autoincrement) { /* ... */ }
); // Same as {$inc: {'some_string': 1} }