How to duplicate array element and modify a value at same time in MongoDb

Question

I have the following document:

[
  {
    "_id": "a",
    "elements": [
      {
        "mode": "ALWAYS",
        "type": "RED",
        "number": "123"
      },
      {
        "mode": "ALWAYS",
        "type": "BLUE",
        "number": "99123124"
      }
    ]
  },
  {
    "_id": "b",
    "elements": [
      {
        "mode": "ALWAYS",
        "type": "RED",
        "number": "123"
      },
      {
        "mode": "ALWAYS",
        "type": "BLUE",
        "number": "123123123"
      }
    ]
  }
]

And I need to duplicate for single entry only the element of type "BLUE" but changing its type to GREEN. How can I accomplish that? I tried without luck using a db.col.push but I couldn't duplicate the specific entry "BLUE" of the array "elements".

This is the expected goal:

[
    {
      "_id": "a",
      "elements": [
        {
          "mode": "ALWAYS",
          "type": "RED",
          "number": "123"
        },
        {
          "mode": "ALWAYS",
          "type": "BLUE",
          "number": "99123124"
        },
        {
          "mode": "ALWAYS",
          "type": "GREEN",
          "number": "99123124"
        }
      ]
    },
    {
      "_id": "b",
      "elements": [
        {
          "mode": "ALWAYS",
          "type": "RED",
          "number": "123"
        },
        {
          "mode": "ALWAYS",
          "type": "BLUE",
          "number": "123123123"
        },
        {
            "mode": "ALWAYS",
            "type": "GREEN",
            "number": "123123123"
        }
      ]
    }
  ]

I don't think this can be accomplished server-side in a single update statement. I believe you will have to use an application to orchestrate at least two commands: One to find records eligible for update, another for creating update operations based on retrieved data. If taking this approach I highly recommend using bulkWrites to speed up the process assuming large datasets. — barrypicker
– barrypicker, Commented Dec 8, 2021 at 17:59

Takis · Accepted Answer · 2021-12-08 19:00:29Z

1

Query

pipeline update requires MongoDB >= 4.2
map and convert the blues to greens
filter to remove those that weren't blues
add those greens to the initial elements array

Test code here

update(
{},
[{"$set": 
   {"elements": 
     {"$concatArrays": 
       ["$elements",
        {"$filter": 
          {"input": 
            {"$map": 
              {"input": "$elements",
               "in": 
               {"$cond": 
                 [{"$eq": ["$$this.type", "BLUE"]},
                  {"$mergeObjects": ["$$this", {"type": "GREEN"}]}, null]}}},
            "cond": {"$ne": ["$$this", null]}}}]}}}])

edited Dec 8, 2021 at 19:00

answered Dec 8, 2021 at 18:45

Takis

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2 Comments

OctavioCega Over a year ago

Thank you, let me test it. It seems to be working

Takis Over a year ago

i changed the query to work even if you have many blues, previous worked for 1 max 1 blue per elements

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