gistfile1.txt

db.uploads.aggregate([
{
    $group: { _id: { userId: '$userId' }, formFields: { $addToSet: '$formFields'} }
},
{
    $unwind:"$formFields"
},
{
    $group: { _id: "$_id", formFieldsCount: { $sum:1} }
}
]);

mongo

db.uploads.mapReduce(
  function () {
    emit( this.userId, this.formFields );
  },
  function (key,values) {

      var reduced = {
      };

      values.forEach(function(value) {
        for ( var k in value ) {
          if ( !reduced.hasOwnProperty(k) )
            reduced[k] = [];

          if ( reduced[k].indexOf( value[k] ) == -1 )
            reduced[k].push(values[k]);
        }

      });

      return reduced;

  },
  {
      "finalize": function(key,value) {

        for (var k in value) {
          if ( Object.prototype.toString.call( value[k] ) !== '[object Array]') {
            var replace = [];
            replace.push( value[k] );
            value[k] = replace;
          }

        }

        return value;
      },
      "out": { "inline": 1 }
  }
)