How to flatten array into a new field in MongoDB

Question

I have a document with an array field named enabled_domain with 1, 2 or more elements. I need to flatten this array into a new field so it will be a string field with the concatenation of all the array field elements separated by comma, for example.

What I have done so far is:

db.myCollection.aggregate(
   [
      { 
          $project: { 
              enabled_domain_2: { 
                  $reduce: {
                      input: "enabled_domain",
                      initialValue: "",
                      in: { 
                          $concat: [ '$$value', '$$this' ] 
                          }
                      }
                  }
              }
          }
   ]
)

... but it does not work.

A sample of myCollection is:

{
    "_id" : ObjectId("56c1fd43e4b0a6078c98108f"),
    "enabled_domain" : [ 
        "A", 
        "B"
    ]
}

/* 2 */
{
    "_id" : ObjectId("5436044fb700a771a18eeac0"),
    "enabled_domain" : [ 
        "A"
    ]
}

How can I make this operation? Thanks in advance.

Answer 1

You just need to add dollar sign in $reduce to reference existing array:

db.myCollection.aggregate(
   [
      { 
          $project: { 
              enabled_domain_2: { 
                  $reduce: {
                      input: "$enabled_domain",
                      initialValue: "",
                      in: { 
                          $concat: [ '$$value', '$$this' ] 
                          }
                      }
                  }
              }
          }
   ]
)

That's what you're missing