code: function(doc, fields) {
  	return Object.keys(fields).reduce((prev, key) => {
  		let val
  
  		if (fields[key].hasOwnProperty('referenceCollection')) {
  			const displayField = fields[key].referenceCollection.defaultField
  
  			if (Array.isArray(doc[key])) {
  				val = doc[key].map(data => {
  					return data[displayField]
  				})
  			} else {
  				val = doc[key][displayField]
  			}
  		} else {
  			val = doc[key]
  		}
  		return Object.assign({}, prev, {
  			[key]: val
  		})
  	}, { _id: doc._id })
  }
docs: Replace populated fields with only the default field value
  @param  {Object} doc
  @param  {Object} fields
  @return {Object}