momgo内嵌数组

相关内容：https://docs.mongodb.com/manual/reference/operator/aggregation-array/

@JsonFormat(pattern = “yyyy-MM-dd”)​ date类型返回值的格式

1，{ $arrayElemAt: [ , ] }​ 指定下标获取数组
Project获取数组中的最后一个

{ "$project": {         "_id": "$_id",         "inpatients": {                    "   $arrayElemAt": ["$inpatients",-1 ]                        },         "id_no": "$id_no",         "name": "$name", } }​​​

{
“project”:{  
“_id”:”_id”,
“inpatients”:{
“arrayElemAt":["inpatients”,-1]
},
“id_no”:”idno”,“name”:”name”,
}
}
2，{ $reverseArray: }​ 数组倒序

db.users.aggregate([   {     $project:      {         name: 1,         reverseFavorites: { $reverseArray: "$favorites" }      }   }])

判断：
The ifNull expression has the following syntax:  
{ifNull: [ , ] }

db.inventory.aggregate(   [      {         $project: {            item: 1,            description: { $ifNull: [ "$description", "Unspecified" ] }         }      }   ])

cond:判断<–布尔表达式–>返回两个指定的返回表达式之一。  
{cond: { if: , then: , else: } }
Or:
{ $cond: [ , , ] }

db.inventory.aggregate(   [      {         $project:           {             item: 1,             discount:               {                 $cond: { if: { $gte: [ "$qty", 250 ] }, then: 30, else: 20 }               }           }      }   ])