MongoDB project part of the array element

Question

I am trying to create a projection where I can take just part of a Array element.

Take this document as a model:

{
    "city_info": {
        "name": "First City"
        "initials": "FC"
    },
    "postal_codes": {
        "ranges": [
            {
                "name": "Range 1",
                "details": "More details",
                "another_object": {
                   (...)
                },
                "codes": [
                    {"code": 1},
                    {"code": 2},
                    {"code": 3}
                ]
            },
            {
                "name": "Range 2",
                "details": "More details 2",
                "another_object": {
                   (...)
                }
                "codes": [
                    {"code": 4},
                    {"code": 5}
                ]
            }
        ]
    }
}

My query would look like {"postal_codes.ranges.codes.code": 3}

Ranges and codes can have hundreads of elements. "another_object" is just a placeholder used for example.

The expected return would be like:

{
    "city_info": {
        "name": "First City"
        "initials": "FC"
    },
    "range": {
        "name": "Range 1",
        "details": "More details",
        "another_object": {
            (...)
        }
    }         
}

In short, I need to get the array element that matches the query too, but do not return it entirely.

It seems that find is not powerfull enough, I would need some kind of aggregation. I have tried to use a match to return only the documents that match the query, but I do not know how to project only part of the array.

Your JSON structure is not valid. "codes": [ "code": 4, "code": 5 ] should be "codes": [{ "code": 4, "code": 5 }]. Isn't? — Gibbs
– Gibbs, Commented Jul 9, 2020 at 16:23

Gibbs · Accepted Answer · 2020-07-09 16:35:39Z

3

You have invalid codes structure. This cannot be inserted in Mongodb

"codes": [
  "code": 1,
  "code": 2,
  "code": 3
]

If you need to keep three codes, valid JSON would be

"codes": [
  1,2,3
]

So sample data:

{
    "_id" : ObjectId("5f074307e96d8884b6bdbdd5"),
    "city_info" : {
        "name" : "First City",
        "initials" : "FC"
    },
    "postal_codes" : {
        "ranges" : [ 
            {
                "name" : "Range 1",
                "details" : "More details",
                "another_object" : {},
                "codes" : [ 
                    1, 
                    2, 
                    3
                ]
            }, 
            {
                "name" : "Range 2",
                "details" : "More details 2",
                "another_object" : {},
                "codes" : [ 
                    4, 
                    5
                ]
            }
        ]
    }
}

Query to get:

db.getCollection('test2').aggregate([
  {
    $unwind: "$postal_codes.ranges"
  },
  {
    $unwind: "$postal_codes.ranges.codes"
  },
  {
    $match: {
      "postal_codes.ranges.codes": 3
    }
  }

])

Output:

/* 1 */
{
    "_id" : ObjectId("5f074307e96d8884b6bdbdd5"),
    "city_info" : {
        "name" : "First City",
        "initials" : "FC"
    },
    "postal_codes" : {
        "ranges" : {
            "name" : "Range 1",
            "details" : "More details",
            "another_object" : {},
            "codes" : 3
        }
    }
}

To avoid codes in the output, you need to use $project as last pipeline

{
 $project:{"postal_codes.ranges.codes":0}
}

answered Jul 9, 2020 at 16:35

Gibbs

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1 Comment

y_159 Over a year ago

in this how do we get the first matching element only? i.e wherever it matches 3 just return it and project it along with other required properties from the document.

Mohammad Faisal · Accepted Answer · 2020-07-09 16:48:50Z

2

You need to use $unwind to convert Array into Objects and then use $match

Try this query,

db.collection.aggregate([
  { "$unwind": "$postal_codes" },
  { "$unwind": "$postal_codes.ranges.codes"},
  { "$match" : { "postal_codes.ranges.codes" : { "$eq": 3 } }},
  { "$project" : { "city_info": 1, "ranges": "$postal_codes.ranges" }}
])

Hope, this may help you.

edited Jul 9, 2020 at 16:48

answered Jul 9, 2020 at 16:33

Mohammad Faisal

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