local variable in lookup aggregate not working-CodePudding

I have two collections, call and contact.

I want to join them and show the contact name in the call record. The below code is what I tried.

ps. when I change "d.v":"$mobile" to "d.v":"some number" it works, but when I use variable is not working, where is my problem ?

{
  from: 'contact',
  let: {"mobile":"$user"},
  pipeline:[
    {
        "$project":{
          d:{$objectToArray:"$data"},
          doc:"$$ROOT",
        }
    },
    {
        "$unwind":{
          path: "$d",
          preserveNullAndEmptyArrays: true
        }
    },
    {
        "$match":{
          "d.v":"$mobile"
          }
    },
],
  as: 'name'
}

CodePudding user response：

As specified in the docs:

To reference variables in pipeline stages, use the "$$" syntax.

...

A $match stage requires the use of an $expr operator to access the variables. The $expr operator allows the use of aggregation expressions inside of the $match syntax.

After changing these two to match the requirements your stage will look like this:

db.call.aggregate([
  {
    "$lookup": {
      from: "contact",
      let: {
        "mobile": "$user"
      },
      pipeline: [
        {
          "$project": {
            d: {
              $objectToArray: "$data"
            },
            doc: "$$ROOT",
            
          }
        },
        {
          "$unwind": {
            path: "$d",
            preserveNullAndEmptyArrays: true
          }
        },
        {
          "$match": {
            $expr: {
              $eq: [
                "$$mobile",
                "$d.v"
              ]
            }
          }
        },
        
      ],
      as: "name"
    }
  }
])

Mongo Playground