CODEWITHSUNDEEP
mongodbfilteraggregation-framework
shohreh
shohreh

Reputation: 526

How to compare two field in aggregation filter in mongoDB

I have a collection like this:

{
   side1:"apple",
   side2:"peach",
   a1:[
      {_id:"apple", info:"test"},
      {_id:"orange", info:"test"},
   ],
   a2:[{_id:"banana", info:"test"},
      {_id:"peach", info:"test"},
      {_id:"apple", info:"test"}]
}

I'm looking for a way to list only subdocuments of a1 and a2 which their _id values are respectively equal to side1 and side2. I mean the result should be the following :

{
   side1:"apple",
   side2:"peach",
   a1:{_id:"apple", info:"test"},
   a2:{_id:"peach", info:"test"}
}

I googled a lot and I have read about $filter, $elemMatch and $where but I could not use them to solve my problem.

Upvotes: 2

Views: 691

Answers (1)

DAXaholic
DAXaholic

Reputation: 35328

An aggregation pipeline like this 

db.getCollection('yourColl').aggregate([
    {
        $project: {
            _id: 0,
            side1: 1,
            side2: 2,
            a1: {
                $filter: {
                    input: "$a1",
                    as: "e",
                    cond: { 
                        $eq: ["$$e._id", "$side1" ]
                    }
                }
            },   
            a2: {
                $filter: {
                    input: "$a2",
                    as: "e",
                    cond: { 
                        $eq: ["$$e._id", "$side2" ]
                    }
                }
            }
        }
    }
])

would have this output for your sample data 

{
    "side1" : "apple",
    "side2" : "peach",
    "a1" : [ 
        {
            "_id" : "apple",
            "info" : "test"
        }
    ],
    "a2" : [ 
        {
            "_id" : "peach",
            "info" : "test"
        }
    ]
}

Upvotes: 2

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