mongodb aggregation pipeline solution for getting record from 2 collections based on the value from an array in one collection

Question

I have two mongoDb collections, one contains data about cards and the other contains data about a field of cards and called list. structure of firstCollection :

{ 
    "cardType":"card", 
    "xyz":"XYZ", 
    "fields":[ 
        {"abc":"abc", "xyz":"XYZ", "inputMethod" : "Entry", "xyz":"xyz"}, 
        {"abc":"abc", "xyz":"XYZ", "inputMethod" : "List", "xyz":"xyz", "ListId":"1234"}
        // ListId will only be present incase of inputMethod=List 
    ] 
}

Structure of secondCollection:

{ "abc":"abc", "xyz":"xyz, "itemId": "1234" }

Now what I want is all the firstCollection where cardType = "card", complete card object and all the items from secondCollection where itemId in (select ListId from firstCollection where fields.inputmethod = "List").

Need to write MongoDB pipeline for this situation. I am quite new to mongo, it can be done using an aggregation pipeline with $loopup but I can write the pipeline. the result I want :

{
    firstCollection:{complete collection },
    secondCollection:[ 
        array of matching records from second collection where 
        secondelement.itemId in(records from array of firstcollection 
        where fields.inputmethod = "List" )
    ]
}

Answer 1

db.first.aggregate([
  {
    $match: {}
  },
  {
    $project: {
      firstCollection: "$$ROOT"
    }
  },
  {
    $lookup: {
      "from": "second",
      "localField": "firstCollection.fields.ListId",
      "foreignField": "itemId",
      "as": "secondCollection"
    }
  }
])

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