Group category and sub categories into one array-CodePudding

I have document in mongoDb database with sub categories and need select sub categories into main object, but can't find way how to do it in MongoDb way Current document:

{
    "title": "main title",
    "subCategories": [
        {
           "title": "sub title 1",
        },
        {
           "title": "sub main title 2",
        },
        {
           "title": "sub main title 3",
        },
    ]
  }

Expecting result:

{[
    {
        "title": "main title"
    },
    {
       "title": "sub title 1",
    },
    {
       "title": "sub main title 2",
    },
    {
       "title": "sub main title 3",
    }
  ]}

CodePudding user response：

As mentioned in the comment, you need to have a key-value pair in the object. Otherwise, it's invalid. To get all things in an array under a single key, try this:

db.collection.aggregate([
  {
    "$project": {
      "subCategories": {
        "$concatArrays": [
          [
            {
              title: {
                "$getField": {
                  "field": "title",
                  "input": "$$ROOT"
                }
              }
            }
          ],
          "$subCategories",
          
        ]
      }
    }
  }
])

Here's the playground link.