HOWTO: Produce mongoDB query string for the fields dynamically from a javascript app

Question

I have a Node.js app with MongoDB
My customers collection schema is like this:

{
    '_id' : 1234341264876876143 , 
    'profile':{
        'name':  'bob',
        'email': 'bob@example.com',
        'DOB':   '13th April 1976'
    }
}

I want to find ONLY the profile.email field of a particular customer from my Node app

var field_name = "email";   // field_name selected programmatically from an array
db.collection('customers').find(
    {'_id':8965698756579084} , 
    {'profile.' + field_name :true}    // expecting it to become 'profile.email' 
)

But, it gives the following error:

    {'profile.' + field_name :true}
                ^
SyntaxError: Unexpected token +
    at Module._compile (module.js:437:25)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:362:17)
    at require (module.js:378:17)

How do i produce the string dynamically for the SPECIFIC fields that i wanna retrieve?

Answer 1

Construct the projection component as a separate step:

var projection = {};
projection['profile.' + field_name] = true;
db.collection('customers').find( {'_id':8965698756579084}, projection );