MongoDB

map-reduce group unique ids by property

db.bettiolo_map_reduce.drop();

db.bettiolo_map_reduce.insert( { id: 1, types : { a: 'foo', b: 'bar', c: 'baz' } } );
db.bettiolo_map_reduce.insert( { id: 1, types : { a: 'foo', b: 'bar', c: 'baz' } } ); // duplicates
db.bettiolo_map_reduce.insert( { id: 2, types : { a: 'foo' } } );
db.bettiolo_map_reduce.insert( { id: 3, types : { b: 'bar' } } );
db.bettiolo_map_reduce.insert( { id: 4 } );
db.bettiolo_map_reduce.insert( { id: 5, types: {} } );

var map = function () {
  for (var type in this.types) { 
	emit(type, this.id);
  }
};

var reduce = function (key, values) {
  return Array.unique(values);
};

var mr = db.bettiolo_map_reduce.mapReduce(map, reduce, {
    out: { "inline" : 1},
    query: { types: { $ne: null } },
    jsMode: true,
});
 
db.bettiolo_map_reduce.drop();

mr.results