MongoDB: How to query arrays in embedded document within several depth?

Question

If this is the one questioned before, please let me know. But I couldn't get exact answer from official docs or stackoverflow.

My mongodb document looks like this.

{
    "_id": ObjectId("614e1b600146c86a7d5cb524"),
    "hash": "hash_1",
    "logs": [
        {
            "address": "address_1",
            "topics": [
                "AAA",
            ],
        },
        {
            "address": "address_2",
            "topics": [
                "CCC",
                "DDD",
            ],
        },
    ],
},

{
    "_id": ObjectId("614e1b470146c86a7d5cacd0"),
    "hash": "hash_2",
    "logs": [
        {
            "address": "address_3",
            "topics": [
                "EEE",
                "FFF"
            ],
        },
    ],
}

I only want to query documents that the first value of topics field of an element in logs field equals with certain value, say "CCC". So can be said like this.

results = []
logs.map((log) => log.topics[0] = "CCC" ? results.push(log))
return results

And I only want elements in logs field with that meet the condition above. Is that possible?

YuTing · Accepted Answer · 2021-09-29 05:53:38Z

2

$match match the condition to reduce document
$project filter array

data

{
    "_id": ObjectId("614e1b600146c86a7d5cb524"),
    "hash": "hash_1",
    "logs": [
      {
        "address": "address_1",
        "topics": [
          "AAA"
        ]
      },
      {
        "address": "address_2",
        "topics": [
          "CCC",
          "DDD"
        ]
      }
    ]
},
{
    "_id": ObjectId("614e1b470146c86a7d5cacd0"),
    "hash": "hash_2",
    "logs": [
      {
        "address": "address_3",
        "topics": [
          "EEE",
          "FFF"
        ]
      }
    ]
}

aggregate

db.collection.aggregate([
  {
    "$match": {
      "logs.topics.0": "CCC"
    }
  },
  {
    $project: {
      hash: 1,
      logs: {
        $filter: {
          input: "$logs",
          as: "log",
          cond: {
            $eq: [
              {
                $arrayElemAt: [
                  "$$log.topics",
                  0
                ]
              },
              "CCC"
            ]
          }
        }
      }
    }
  }
])

result

{
  "_id": ObjectId("614e1b600146c86a7d5cb524"),
  "hash": "hash_1",
  "logs": [
    {
      "address": "address_2",
      "topics": [
        "CCC",
        "DDD"
      ]
    }
  ]
}

mongoplayground

edited Sep 29, 2021 at 5:53

answered Sep 29, 2021 at 3:21

YuTing

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3 Comments

kimiro Over a year ago

Thanks for your quick and correct answer. Btw, I have one more question. What if I want to apply a condition for second element of topics array. It seems like there is no expression like $eq: [{$second: "$$log.topics"},"DDD"] And second is not the last one. There are other elements folllowing like "EEE", "FFF"...

YuTing Over a year ago

I update my answer. Using $arrayElemAt can get second or third element.

kimiro Over a year ago

You saved me a day! Thanks a lot @YuTing. I finally was able to get exact elements I wanted with $and expression and your answer.

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