mongo_join

db.users.aggregate([
    { "$group" : { "__tmp0" : { "$push" : "$$ROOT"} , "_id" : "$_id"}},
    { "$project" : { "_id" : "$_id" , "value" : { "left" : "$__tmp0" , "right" : { "$literal" : [ ]} , "_id" : "$_id"}}},
    { "$out" : "tmp.gen_0"}
  ],
  { allowDiskUse: true })
db.posts.mapReduce(
  function () {
    emit.apply(null, (function (key, value) {
        return [(value != null) ? value.user : undefined, {
          "left": [],
          "right": [value]
        }];
      })(this._id, this));
  },
  function (key, values) {
    var result = {
      "left": [],
      "right": []
    };
    values.forEach(function (value) {
        result.left = result.left.concat(value.left);
        result.right = result.right.concat(value.right);
      });
    return result;
  },
  { "out" : { "reduce" : "tmp.gen_0" , "nonAtomic" : true}})
db.tmp.gen_0.aggregate([
    { "$match" : { "value.left" : { "$not" : { "$size" : 0}} , "value.right" : { "$not" : { "$size" : 0}}}},
    { "$unwind" : "$value.left"},
    { "$unwind" : "$value.right"},
    { "$group" : { "cnt" : { "$sum" : { "$literal" : 1}} , "displayName" : { "$push" : "$value.left.displayName"} , "_id" : "$value.right.user"}},
    { "$unwind" : "$displayName"},
    { "$group" : { "__tmp1" : { "$first" : "$$ROOT"} , "_id" : { "0" : "$cnt" , "1" : "$displayName"}}},
    { "$project" : { "cnt" : "$__tmp1.cnt" , "displayName" : "$__tmp1.displayName" , "_id" : false}},
    { "$out" : "tmp.gen_1"}
  ],
  { allowDiskUse: true })
db.tmp.gen_0.drop()
db.tmp.gen_1.find()