$concat(聚合)
定义
举例
请考虑包含以下文档的 inventory
集合:
{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "product 1" } { "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2" } { "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }
以下操作使用 $concat
操作符和“-”分隔符将 item
字段与 description
字段连接起来。
db.inventory.aggregate( [ { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } } ] )
操作返回以下结果:
{ "_id" : 1, "itemDescription" : "ABC1 - product 1" } { "_id" : 2, "itemDescription" : "ABC2 - product 2" } { "_id" : 3, "itemDescription" : null }