Count and Update nested array in mongodb

Question

I am new to using mongo db and have been trying to figure out a way to get count of an nested array in mongo db collection which looks like follows.

    {
      city_id: 'C1',
      city_name: "C1Name",
      wards: [{
      ward_id: "W1",
      ward_number: "1",
      areas: [{
      area_id: "a1",
      area_name: "a1Name"
    },
    {
      area_id: "a2",
      area_name: "a2Name"
    }]
  }, 
  {
    ward_id: "W2",
    ward_number: "2",
    areas: [{
      area_id: "a21",
      area_name: "a21Name"
    }, {
      area_id: "a22",
      area_name: "a22Name"
    }]
  }]
},
    {
      city_id: 'C2',
      city_name: "C2Name",
      wards: [{
      ward_id: "W21",
      ward_number: "21",
      areas: [{
        area_id: "aw21",
        area_name: "aw21Name"
    },{
        area_id: "aw22",
        area_name: "aw22Name"
    }]
  },{
      ward_id: "W132",
      ward_number: "132",
      areas: [{
      area_id: "a132",
      area_name: "a132Name"
    }, {
      area_id: "a1332",
      area_name: "a2112Name"
    }]
  }]
}

now here I want number of areas in city_details where city_id=C1 and ward_id=W1

I used aggregation as

    db.city_details.aggregate(
      {$match:{"city_id":"C1"}},
      {$project:{"wards":"$wards"}},
      {$unwind:"$wards"},   
      {$project{"ward":"$wards.id","area":"$wards.areas"}},
      {$match:{ward:"W1"}},
      {$project:{$count:{"area"}}
    )

This should get me count as 2 but it isn't working.

Also how do I update these areas arrays ?

score 0 · Accepted Answer · answered Aug 16 '17 at 09:34

The reason your query doesn't work is because {$match:{ward:"W1"}} can't find W1 within the previous result.

The Result before running {$match:{ward:"W1"}} looks like this:

{
    "area" : [ 
        {
            "area_id" : "a1",
            "area_name" : "a1Name"
        }, 
        {
            "area_id" : "a2",
            "area_name" : "a2Name"
        }
    ]
},
{
    "area" : [ 
        {
            "area_id" : "a21",
            "area_name" : "a21Name"
        }, 
        {
            "area_id" : "a22",
            "area_name" : "a22Name"
        }
    ]
}

There is no W1 to find.

Instead you can use a query like this:

db.city_details.aggregat([
  {$match: {"city_id": "C1"}},
  {$project: {"ward": {$filter: {"input": "$wards", "as": "ward", "cond": { $eq: ["$$ward.ward_id", "W1"]}}}}},
  {$unwind:"$ward"},
  {$project: {"size": {$size: "$ward.areas"}}}
])

Regarding to your second question. It's not that easy, this post will give you more to read about updating nested arrays in mongodb.

Hii, thanks Julian your query did the job but I don't understand the use of two $ signs in the $eq operator — TGW, Aug 16 '17 at 10:06
More details can be found in the [mongodb docu](https://docs.mongodb.com/manual/reference/aggregation-variables/). But in short, with _$$_ you can access the current value from _ward_. — Julian Hilbers, Aug 16 '17 at 11:06

Count and Update nested array in mongodb

1 Answers1