Add pipeline stage in the arguments of another pipeline - MongoDB

Question

Is it possible to add a pipeline to the arguments of another pipeline in mongodb?

An example of the current result that I am getting: MongoPlayground

Is it possible to project the emp table to only name and email field before adding it to the lookup pipeline as an argument? The result that I want:

[
  {
    "_id": ObjectId("610bce417b0c4008346547bc"),
    "employees": [
      {
        "email": "[email protected]",
        "name": "xyz"
      }
    ],
    "name": "Chicago",
    "number": 10
  }
]

Answer 1

You are almost there. What you seek is the pipeline version of $lookup:

db.dept.aggregate([
    {$lookup: {"from": "emp",
           let: { did: "$_id" },
           pipeline: [
               {$match: {$expr: {$eq: [ "$_id", "$$did" ]} }},
               {$project: {
                   _id:false,
                   email:true,
                   name:true
           }}
       ],
       as: "employees"
   }}
]);

which will yield:

{
    "_id" : 0,
    "name" : "Chicago",
    "number" : 10,
    "employees" : [
        {
            "name" : "xyz",
            "email" : "[email protected]"
        }
    ]
}