mungedb-aggregate/lib/pipeline/accumulators/FirstAccumulator.js

"use strict";

/** 
 * Constructor for FirstAccumulator, wraps SingleValueAccumulator's constructor and adds flag to track whether we have started or not
 * @class FirstAccumulator
 * @namespace mungedb-aggregate.pipeline.accumulators
 * @module mungedb-aggregate
 * @constructor
 **/
var FirstAccumulator = module.exports = function FirstAccumulator(){
	base.call(this);
	this.started = 0; //TODO: hack to get around falsy values making us keep going
}, klass = FirstAccumulator, base = require("./SingleValueAccumulator"), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

// PROTOTYPE MEMBERS
proto.getOpName = function getOpName(){
	return "$first";
};

proto.getFactory = function getFactory(){
	return klass;	// using the ctor rather than a separate .create() method
};

/** 
 * Takes a document and returns the first value in the document
 * @param {Object} doc the document source
 * @return the first value
 **/
proto.evaluate = function evaluate(doc){
	if (this.operands.length != 1) throw new Error("this should never happen");

	// only remember the first value seen
	if (!base.prototype.getValue.call(this) && this.started === 0) {
		this.value = this.operands[0].evaluate(doc);
		this.started = 1;
	}

	return this.value;
};