// Requires Node.js v6+ and a MongoDB library that supports promises (e.g. monk)

/* EXAMPLE USAGE:
const monk = require('monk');
const dbUtils = require('./dbUtils.js');

const db = monk('localhost:27017/monkey');
const collection = db.get('contentItems');
const newValue = 'WHATEVER';

dbUtils.updateFieldInArrayOfObjects(db, collection, 'tracks', 'checked', newValue)
  .then(() => db.close())
  .catch(err => console.error(err));
*/

// For all documents in a collection, that contain a field called arrayField,
// which stores an array of objects, where objects in that array contain a field
// called objectField - this function will update the value of objectField to newValue
function updateFieldInArrayOfObjects(db, collection, arrayField, objectField, newValue) {

  return collection
    // Get all documents from collection
    .find({}, { [arrayField]: 1 })
    // Iterate each document
    .each(doc => {

      // Clone the arrayField array
      let arrayFieldClone = Array.from(doc[arrayField]);

      // Update all objectFields in arrayFieldClone with new value
      arrayFieldClone.forEach(item => item[objectField] = newValue);

      // Save arrayFieldClone to dB, replacing existing arrayField
      collection
        .update({ _id: doc._id }, { $set: { [arrayField]: arrayFieldClone } })
        .then(() => console.log(`Updated all ${arrayField}.${objectField} values to ${newValue} in doc ${doc._id}`));
    });
}

// Export the function for external use
module.exports = {

  updateFieldInArrayOfObjects
}