如何計算mongo集合中的數組長度？

Question

我有以下蒙戈集合

[{ 
"_id" : ObjectId("543fa67e9672ec37ebe3d028"), 
"customerId" : "5429aed703646b52def71736", 
"tagName" : "abc", 
"tagMembers" : [ 
    "111", 
    "112", 
    "123" 
] 
}, 
{ 
"_id" : ObjectId("543fa67e9672ec37ebe3d029"), 
"customerId" : "5429aed703646b52def71736", 
"tagName" : "pqr", 
"tagMembers" : [ 
    "111", 
    "112", 
    "123" 
] 
}] 

結構我要計算每個tagMember的計數。

如何使用mongo來計算它？

是否有可能在不使用聚合的情況下計數？

Answer 1

使用聚合框架和$size操作：

> db.test.aggregate([ 
    { "$project" : { "_id" : 0, "tagMembersLength" : { "$size" : "$tagMembers" } } } 
]) 
{ "tagMembersLength" : 3 } 
{ "tagMembersLength" : 3 }