How to aggregate distinct items in mongo db to separate array?

I have an API endpoint that checks the mongo db and returns in a following structure for example:

{    "Name": "Test"    "Location": "Whatever",    "customerServices": [        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "West",          "name": "test"        },        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "West",          "name": "test"        },        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "East",          "name": "test"        }    ]  }  

What I want to do is to add aggregation to the pipeline that would create additional field-serviceCounties and include distinct serviceCounty values from the customerServices array. Like the following:

{    "Name": "Test"    "Location": "Whatever",    "customerServices": [        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "West",          "name": "test"        },        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "West",          "name": "test"        },        {          "id": "test",          "cusId": "test",          "serviceAdr": "test",          "serviceCounty": "East",          "name": "test"        }    ],    "serviceCounties": [      {        "countyName":"East"      },      {        "countyName":"West"      }  ]   }  

I have tried the following, but it didn't work. There something I'm missing or doing wrong:

                'serviceCounties': {                      '$reduce': {                          'input': '$services.event.services',                          'initialValue': [],                          'in': [{                              "countyName": { '$concatArrays': ["$$value.serviceCounty", "$$this"] }                          }]                      }                  }  

Any ideas how to do it ?

https://stackoverflow.com/questions/65803981/how-to-aggregate-distinct-items-in-mongo-db-to-separate-array January 20, 2021 at 02:12PM