Insert element into nested arrays in MongoDB

Question

I am new to mongoDB. I am having some trouble in updating the records in mongoDB collection.

How to add elements into array likes into the embedded record

I have a embedded collection like:

{
  "_id": "iL9hL2hLauoSimtkM",
  "title": "Some Topic",
  "followers": [
    "userID1",
    "userID2",
    "userID3"
  ],
  "comments": [
    {
      "comment": "Yes Should be....",
      "userId": "a3123",
      "likes": [
        "userID1",
        "userID2"
      ]
    },
    {
      "comment": "No Should not be....",
      "userId": "ahh21",
      "likes": [
        "userID1",
        "userID2",
        "userID3"
      ]
    }
  ]
}

I want to update the record as

{
  "_id": "iL9hL2hLauoSimtkM",
  "title": "Some Topic",
  "followers": [
    "userID1",
    "userID2",
    "userID3"
  ],
  "comments": [
    {
      "comment": "Yes Should be....",
      "userId": "a3123",
      "likes": [
        "userID1",
        "userID2",
        "userID3" // How to write query to add this element.
      ]
    },
    {
      "comment": "No Should not be....",
      "userId": "ahh21",
      "likes": [
        "userID1",
        "userID2",
        "userID3"
      ]
    }
  ]
}

Please provide the query to add the element shown in comment. Thank you.

gender_madness · Accepted Answer · 2016-03-20 12:03:26Z

26

Two possibilities here:

Since you don't have an unique identifier for the comments, the only way to update an specific item on the comments array is to explicitly indicate the index you are updating, like this:
```
db.documents.update(
  { _id: "iL9hL2hLauoSimtkM"},
  { $push: { "comments.0.likes": "userID3" }}
);
```
If you add an unique identifier for the comments, you can search it and update the matched item, without worrying with the index:
```
db.documents.update(
  { _id: "iL9hL2hLauoSimtkM", "comments._id": "id1"},
  { $push: { "comments.$.likes": "userID3" }}
);
```

answered Mar 20, 2016 at 12:03

gender_madness

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1 Comment

python_kaa Over a year ago

just wonder if it were possible to update multiple array items with one update? I have a case where it can be up to 50 updates per document.

BRAHIM Kamel · Accepted Answer · 2016-03-20 14:34:41Z

2

You can try $addToSet which adds elements to an array only if they do not already exist in the set.

db.topics.update(
   { _id: "iL9hL2hLauoSimtkM" },
   { $addToSet: {  "comments.0.likes": "userId3" } }
)

answered Mar 20, 2016 at 14:34

BRAHIM Kamel

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1 Comment

Pooya Estakhri Over a year ago

Finally an answer that actually works

noirsociety · Accepted Answer · 2020-07-16 15:12:10Z

0

Try this:

db.<collection>.updateOne(
   {_id:"iL9hL2hLauoSimtkM",comments:{$elemMatch:{userId:'a3123'}}},
   {$push:{'comments.$.likes':'userID3'}})

$ operator is equivalent to the filter

$ === {_id:"iL9hL2hLauoSimtkM",comments:{$elemMatch:{userId:'a3123'}}}

nevertheless, we are basically pushing "userID3" into

[1] the "likes" array
[2] inside the specified element === $
[3] which belonging to the "comments" array

edited Jul 16, 2020 at 15:12

answered Jul 15, 2020 at 15:08

noirsociety

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