MongoDB: Check for array for object and if exists return true, otherwise false

Question

I'm trying to query an array within a collection and project the value "true" if the object exists within the array. If the object doesn't exist within the array, project "false". I am working within MongoDB and I'm not too familiar with it.

In my scenario, I have two collections that I am working with. I am aggregating a collection of 'staff' members and I am performing a $lookup function to a collection of 'businesses'. Within 'businesses' I have an array of the businesses capabilities.

For example, I have staff collection

staff = [
  ...
  {_id: 1, businessId: 11},
  {_id: 2, businessId: 22},
  ....
]

Businesses collection

businesses = [
  ...
  {_id: 11, capabilities: ["claiming", "pushing"]},
  {_id: 22, capabilities: ["claiming", "popping"]},
  ....
]

And have a $lookup setup like

db.getCollection('staff').aggregate([
    {
        $lookup:
            {
                from: "businesses",
                localField: "businessId",
                foreignField: "_id",
                as: "business_Info"
            }
    },

How can I $project per staff member to a value like $canClaim: true, if "claiming" appears within "capabilities"?

Ashh · Accepted Answer · 2019-05-10 07:31:04Z

8

You can use $in aggregation operator to check whether the array contains the value or not.

db.getCollection("staff").aggregate([
  { "$lookup": {
    "from": "businesses",
    "localField": "businessId",
    "foreignField": "_id",
    "as": "business_Info"
  }},
  { "$unwind": "$business_Info" },
  { "$addFields": {
    "canClaim": { "$in": ["claiming", "$business_Info.capabilities"] }
  }}
])

MongoPlayground

edited May 10, 2019 at 7:31

answered May 10, 2019 at 7:05

Ashh

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2 Comments

Maandeep Over a year ago

Awesome! Thanks I will try this soon. Out of curiousity, does the $unwind function affect calling fields in $business_Info?

Ashh Over a year ago

No it will only Deconstructs your array field

Rubin Porwal · Accepted Answer · 2019-05-11 06:42:02Z

db.getCollection("staff").aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $lookup: // Equality Match
            {
                from: "businesses",
                localField: "businessId",
                foreignField: "_id",
                as: "businessInfo"
            }

            // Uncorrelated Subqueries
            // (supported as of MongoDB 3.6)
            // {
            //    from: "<collection to join>",
            //    let: { <var_1>: <expression>, …, <var_n>: <expression> },
            //    pipeline: [ <pipeline to execute on the collection to join> ],
            //    as: "<output array field>"
            // }
        },

        // Stage 2
        {
            $project: {
                businessId: 1,
                businessInfo: {
                    $arrayElemAt: ['$businessInfo', 0]
                }
            }
        },

        // Stage 3
        {
            $addFields: {
                "businessInfo.canClaim": {
                    $cond: {
                        if: {
                            $gte: [{
                                $indexOfArray: ["$businessInfo.capabilities", 'claiming']
                            }, 0]
                        },
                        then: true,
                        else: false
                    }
                }
            }
        },

    ]



);

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