How to concatenate string array field to show as one string if its an array

Question

Following are two documents in my collection:

{
    "_id": {
        "$oid": "5f48e358d43721376c397f54"
    },
    "heading": "this is heading",
    "tags": ["tag1","tag2","tag3"],
    "categories": ["projA", "projectA2"],
    "content": ["This", "is", "the", "content", "of", "the", "document"],
    "timestamp": 1598612312.506219,
    "lang": "en"
}

and

{
    "_id": {
        "$oid": "5f48e358d43721376c397f53"
    },
    "heading": "this is heading",
    "tags": "tag1,tag2,tag3",
    "categories": "projA, projectA2",
    "content": "This is the content of the document",
    "timestamp": 1598612312.506219,
    "lang": "en"
}

I want to write a query that concatenates the content element for first document and prints the second as is(as it is not an array).

I am using the following script to concatenate but it is giving error on the second document saying that it needs to be an array and it is not. I cannot figure out how to use switch statement or nested if conditions in this scenario:

    "$addFields": {
  content: { 
      "$reduce": {
        "input": "$content",
        "initialValue": "",
        "in": {
          "$cond": {
            "if": { "$eq": [ { "$indexOfArray": [ "$content", "$$this" ] }, 0 ] },
            "then": { "$concat": [ "$$value", "$$this" ] },
            "else": { "$concat": [ "$$value", "\n", "$$this" ] }
          }    
        }
      }        
    }
}

I am very new to mongo DB so any help will be appreciated.

turivishal · Accepted Answer · 2020-11-13T12:15:14.813

You can try,

$cond to check content's type is array or not, $reduce to concat content string and $trim to remove white space

db.collection.aggregate([
  {
    $addFields: {
      content: {
        $cond: [
          { $eq: [{ $type: "$content" }, "array"] },
          {
            $trim: {
              input: {
                $reduce: {
                  input: "$content",
                  initialValue: "",
                  in: { $concat: ["$$value", " ", "$$this"] }
                }
              }
            }
          },
          "$content"
        ]
      }
    }
  }
])

Playground

How to concatenate string array field to show as one string if its an array

1 Answers1