Asymmetrik/mean2-starter

'use strict';

var through2 = require('through2'),
    path = require('path'),
    stringify = require('csv-stringify'),
    jsonpath = require('JSONPath'),
    pipe = require('multipipe'),

    deps = require(path.resolve('./src/server/dependencies.js')),
    logger = deps.logger;

/**
 * Creates a stream that accepts objects as input and returns a serialized CSV file.
 * The objects in the stream can come directly from Mongo.
 *
 * @param {Array{Object}} columns An array defining the columns to output in the CSV, in order.
 *   Each object must contain:
 *     - key: A JSONPath selector to get the value for this column from the object
 *     - title: The title to use in the CSV header for the column
 *     - callback: Optionally, a function to do further processing of the value.  It must accept a value
 *         and return a value.
 *
 * @returns {Stream} A stream that can be piped to the HTTP response.
 */
module.exports = function(columns) {

    // Create a stream to turn Mongo records into CSV rows
    var stream = through2.obj(function (chunk, enc, callback) {
        var row = [];

        // Turn Mongo models into actual objects so JSONPath can work with them
        if (null != chunk.toObject) {
            chunk = chunk.toObject();
        }

        columns.forEach(function (column) {
            if (column.hasOwnProperty('key')) {
                // Get the value from the object using jsonpath
                var value = jsonpath.eval(chunk, '$.' + column.key);

                // Get the first returned value
                if (value.length > 0) {
                    value = value[0];
                }
                else {
                    value = null;
                }

                // Invoke any callback associated with the column
                if (column.hasOwnProperty('callback')) {
                    value = column.callback(value);
                }

                // Emit a blank column rather than null
                if (null == value) {
                    value = '';
                }
                row.push(value);
            }
        });

        // Emit the row to the output stream, piped to the CSV stringifier
        callback(null, row);
    });

    // Parse the columns array into a format the CSV stringify module is expecting
    var csvColumns = [];
    columns.forEach(function(value) {
        if (value.hasOwnProperty('title')) {
            csvColumns.push(value.title);
        }
    });

    // Assemble the CSV headers and stream the CSV response back to the client
    var csv = stringify({
        header: true,
        columns: csvColumns
    });

    // Create an output stream piping the parsing stream to the CSV stream
    var out = pipe(stream, csv);

    out.on('error', function(err) {
        logger.err(err, 'Failed to create CSV');
    });

    return out;
};