mungedb-aggregate/lib/pipeline/expressions/ConstantExpression.js

"use strict";

/** 
 * Internal expression for constant values 
 * @class ConstantExpression
 * @namespace mungedb-aggregate.pipeline.expressions
 * @module mungedb-aggregate
 * @constructor
 **/
var ConstantExpression = module.exports = function ConstantExpression(value){
	if (arguments.length !== 1) throw new Error("args expected: value");
	this.value = value;	//TODO: actually make read-only in terms of JS?
	base.call(this);
}, klass = ConstantExpression, base = require("./Expression"), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

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

/**
 * Get the constant value represented by this Expression.
 * @method getValue
 * @returns the value
 **/
proto.getValue = function getValue(){	//TODO: convert this to an instance field rather than a property
	return this.value;
};

proto.addDependencies = function addDependencies(deps, path) {
	// nothing to do
};

/**
 * Get the constant value represented by this Expression.
 * @method evaluate
 **/
proto.evaluate = function evaluate(doc){
	return this.value;
};

proto.optimize = function optimize() {
	return this; // nothing to do
};

proto.toJSON = function(isExpressionRequired){
	return isExpressionRequired ? {$const: this.value} : this.value;
};

//TODO:	proto.addToBsonObj   --- may be required for $project to work -- my hope is that we can implement toJSON methods all around and use that instead
//TODO:	proto.addToBsonArray