mongodb 数组中文档的长度 java

[dream天空]

{
"_id" : 3,
"repos" : {
"com" : [
{ "a" : 16555, "t" : 141, "d" : 7633 } ,
{ "a" : 14358, "t" : 121, "d" : 7133 } ,
{ "a" : 12356, "t" : 108, "d" : 2137 } ,
],
"self" : [
{
"forks_count" : 54,
"id" : 7370053,
"rank_score" : 0.0006762191350350863
},
{
"forks_count" : 54,
"id" : 7370053,
"rank_score" : 0.0006762191350350863
}]
}
}
如何求self中数组文档的个数？
各路大神指导

[大雨将至]

$size 命令

[yuzhongzhu123]

楼主：
1楼正解
db.collection.aggregate([{$project:{_id:1,cnt:{$size:'$数组字段'}}}])