concatenate string values in array to a single field in DocumentDB

Question

Suppose that I have a series of documents with the following format:  
  
{  
    "_id": "3_0",  
    "values": $$"Test1", "Test2"$$  
}  
  
and I would like to obtain a projection of the array's values concatenated in a single field:  
  
{  
    "_id": "3_0",  
    "values": "Test1_Test2"  
}  
  
I could use following when i use mongodb, however, reduce is not supported in documentDB and I could not get this to work.   
db.collection.aggregate(\[  
  { "$addFields": {  
    "values": {   
      "$reduce": {  
        "input": "$values",  
        "initialValue": "",  
        "in": {  
          "$cond": {  
            "if": { "$eq": \[ { "$indexOfArray": \[ "$values", "$$this" ] }, 0 ] },  
            "then": { "$concat": \[ "$$value", "$$this" ] },  
            "else": { "$concat": \[ "$$value", "_", "$$this" ] }  
          }      
        }  
      }          
    }  
  }}  
])  
  
Could you please tell me if there's a workout. Thank you  
  
Edited by: heshananupama on Feb 1, 2021 11:41 AM  
  
Edited by: heshananupama on Feb 1, 2021 11:41 AM  
  
Edited by: heshananupama on Feb 1, 2021 11:41 AM  
  
Edited by: heshananupama on Feb 1, 2021 11:42 AM

Answer

DocumentDB does support  $reduce.  for

```
rs0:PRIMARY> db.testc.find()
{ "_id" : "3_0", "values" : [ "Test1", "Test2" ] }
rs0:PRIMARY> db.testc.aggregate([ { "$addFields": { "values": { "$reduce": { "input": "$values", "initialValue": "", "in": { "$cond": { "if": { "$eq": [ { "$indexOfArray": [ "$values", "$$this" ] }, 0 ] }, "then": { "$concat": [ "$$value", "$$this" ] }, "else": { "$concat": [ "$$value", "_", "$$this" ] } } } } } }} ])
{ "_id" : "3_0", "values" : "Test1_Test2" }
rs0:PRIMARY> db.version()
4.0.0

```

concatenate string values in array to a single field in DocumentDB

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