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.

Could you add sample document from myCollection ?

mickl
– mickl

2018-03-29 19:28:37 +00:00
Commented Mar 29, 2018 at 19:28 — mickl
– mickl, Commented Mar 29, 2018 at 19:28
@mickl Just updated the question. Thanks.

Lucas Rezende
– Lucas Rezende

2018-03-29 19:30:44 +00:00
Commented Mar 29, 2018 at 19:30 — Lucas Rezende
– Lucas Rezende, Commented Mar 29, 2018 at 19:30

mickl · Accepted Answer · 2018-03-29 19:32:47Z

2

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

answered Mar 29, 2018 at 19:32

mickl

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3 Comments

Lucas Rezende Over a year ago

I added the dollar sign but still got an error. I think because $reduce operator does not work for version 3.2.19. Is there any workaround?

mickl Over a year ago

@LucasRezende I'm afraid you have to upgrade your MongoDB server to 3.6

Lucas Rezende Over a year ago

Hum... I think so. Thank you for your help!

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