Node.js mongodb 聚合查询?

Question

{ "_id" : 1, "item" : "abc", "tag" : "tag1"},
{ "_id" : 2, "item" : "jkl", "tag" : "tag2"},
{ "_id" : 3, "item" : "xyz", "tag" : "tag3"},
{ "_id" : 4, "item" : "xyz", "tag" : "tag4"},
{ "_id" : 5, "item" : "abc", "tag" : "tag5"}

对数据进行分组，期望得到以下结果应该如何查询

{ "_id" : "abc", "tags" : ['tag1','tag5']},
{ "_id" : "jkl", "tags" : ['tag2']},
{ "_id" : "xyz", "tags" : ['tag3','tag4']},