How could we merge nested arrays of subdocuments(Array within Array) in MongoDB

Question

I would like to combine arrays within arrays in MongodB

for example, test collection has documents in following formats

**{
cmp_id :1
depts : [{"dept_id":1, emps:[1,2,3]}, {"dept_id":2, emps:[4,5,6]}, {"dept_id":2, emps:[7,8,9]} ]
}**

I need following output, How can I?

***{
cmp_id :1,
empids : [1,2,3,4,5,6,7,8,9]
}***

Ashh · Accepted Answer · 2020-04-05 07:28:57Z

1

You can use below aggregation

db.collection.aggregate([
  {
    $project: {
      depts: {
        $reduce: {
          input: "$depts",
          initialValue: [],
          in: {
            $concatArrays: [
              "$$value",
              "$$this.emps"
            ]
          }
        }
      }
    }
  }
])

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answered Apr 5, 2020 at 7:28

Ashh

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Rubin Porwal · Accepted Answer · 2020-04-05 14:30:59Z

0

db.collection.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $unwind: {
                path: "$depts"
            }
        },

        // Stage 2
        {
            $unwind: {
                path: "$depts.emps"
            }
        },

        // Stage 3
        {
            $group: {
                _id: '$cmp_id',
                empids: {
                    $push: '$depts.emps'
                }

            }
        },

    ]



);

answered Apr 5, 2020 at 14:30

Rubin Porwal

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