MongoDB Convert Array of Objects to Single Object

Question

Using a MongoDB aggregate query, how do I convert an array of documents to a single document. The array can have N number of documents.

Before

"loop" : [
    {
        "field1" : "1"
    }, 
    {
        "field2" : "2", 
        "field3" : "3", 
    }, 
    {
        "field4" : "4", 
    }, 
    {
        "field5" : "5", 
        "field6" : "6"
    }
]

After

"loop" : {
    "field1" : "1",
    "field2" : "2", 
    "field3" : "3", 
    "field4" : "4", 
    "field5" : "5", 
    "field6" : "6"
}

s7vr · Accepted Answer · 2019-01-19 03:33:42Z

4

You can use below aggregation in 3.6 and above.

db.colname.aggregate(
 [{"$project":{
    "loop":{
      "$reduce":{
        "input":"$loop",
        "initialValue":{},
        "in":{"$mergeObjects":["$$value","$$this"]
        }
      }
    }
 }}]
)

answered Jan 19, 2019 at 3:33

s7vr

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1 Comment

Yan Khonski Over a year ago

Please, help stackoverflow.com/questions/55013224/…

Matthew · Accepted Answer · 2019-01-19 16:23:15Z

3

Based on Veeram's answer above using $mergeObjects, here's a more concise query using $project.

Query

db.loop.aggregate(
    [{
        "$project": {
            "loop": { $mergeObjects: "$loop" }
        }
    }]
)

Result

{ 
    "loop" : {
        "field1" : "1", 
        "field2" : "2", 
        "field3" : "3", 
        "field4" : "4", 
        "field5" : "5", 
        "field6" : "6"
    }
}

answered Jan 19, 2019 at 16:23

Matthew

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1 Comment

salerokada Over a year ago

This also works for older MongoDb version. Specifically, I have 3.0.6.

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