如何使用java在mongoDB上执行连接操作

Question

我有两张桌子

1. Department collection
2. Employee collection

    

Both the table have unique _id. I have stored multiple employee ids on Department collection.

Employee Collection:
{
 _id:"EMP1",
name:"Arjun Singh",
designation:"developer"
},
{
 _id:"EMP2",
name:"Hitesh",
designation:"VTS support"
}


Department Collection:
{
 _id:"3",
name:"XYZ Department",
employeeList:["EMP1","EMP2"]
}

执行mongo db连接后，我需要以下格式的数据

{
_id:"3",
name: "XYZ Department",
 employeeList:[
{
    _id:"EMP1",
    name:"Arjun Singh",
    designation:"developer"
},
{
    _id:"EMP2",
    name:"Hitesh",
    designation:"VTS support"
}
]
}

请帮我讲解如何在mongoDb中执行联接

Answer 1

这将产生您所寻求的输出。请注意，通过将as字段设置为employeeList，它会使用展开的＆＃34;加入＆＃34;覆盖原始列表。数据：

db.department.aggregate([
{$lookup: { from: "employee", localField: "employeeList", foreignField: "_id", as: "employeeList"}}
                ]);