连接数组值

Question

如何在mongoDb聚合查询中连接数组值

{
_id: 1,
name:"abc"
desc:"test"
tags:[a,b,c]
},

{
_id: 2,
name:"xyc"
desc:"test1"
tags:[d,s,c]
}

结果将是

{
_id: 1,
name:"abc"
desc:"test"
tags:a,b,c
},

{
_id: 2,
name:"xyc"
desc:"test1"
tags:d,s,c
}