Is it possible to group by multiple dimensions in crossfilter?

Question

For Example If we have data for books, authors and date information. Can we build a crossfilter for how many books are present for author per month?

Answer 1

I want to update an old answer with a new work around described in: https://github.com/dc-js/dc.js/pull/91

This performance hasn't been tested on large data-sets

  var cf = crossfilter([
  { date:"1 jan 2014", author: "Mr X", book: "Book 1" },
  { date:"2 jan 2014", author: "Mr X", book: "Book 2" },
  { date:"3 feb 2014", author: "Mr X", book: "Book 3" },
  { date:"1 mar 2014", author: "Mr X", book: "Book 4" },
  { date:"2 apr 2014", author: "Mr X", book: "Book 5" },
  { date:"3 apr 2014", author: "Mr X", book: "Book 6"},
  { date:"1 jan 2014", author: "Ms Y", book: "Book 7" },
  { date:"2 jan 2014", author: "Ms Y", book: "Book 8" },
  { date:"3 jan 2014", author: "Ms Y", book: "Book 9" },
  { date:"1 mar 2014", author: "Ms Y", book: "Book 10" },
  { date:"2 mar 2014", author: "Ms Y", book: "Book 11" },
  { date:"3 mar 2014", author: "Ms Y", book: "Book 12" },
  { date:"4 apr 2014", author: "Ms Y", book: "Book 13" }
  ]);

  var dimensionMonthAuthor = cf.dimension(function (d) {
    var thisDate = new Date(d.date);
    //stringify() and later, parse() to get keyed objects
    return JSON.stringify ( { date: thisDate.getMonth() , author: d.author } ) ;
  });

  group = dimensionMonthAuthor.group();
  //this forEach method could be very expensive on write.
  group.all().forEach(function(d) {
    //parse the json string created above
    d.key = JSON.parse(d.key);
  });

  return group.all()

Results in:

[ { key: { date: 0, author: 'Mr X' },
    value: 2 },
  { key: { date: 0, author: 'Ms Y' },
    value: 3 },
  { key: { date: 1, author: 'Mr X' },
    value: 1 },
  { key: { date: 2, author: 'Mr X' },
    value: 1 },
  { key: { date: 2, author: 'Ms Y' },
    value: 3 },
  { key: { date: 3, author: 'Mr X' },
    value: 2 },
  { key: { date: 3, author: 'Ms Y' },
    value: 1 } ]