mongodb-idempotent-array-push.md

Idempotent array push

db.myRecords.findAndModify(
    {
      _id: recordId, 
      itemArray: {$not: { $elemMatch: { itemId: newItem.id } }}
    },
    { 
      $push: {itemArray: newItem}
    }
);

We have to be careful about the matching condition and the content we want to push into the array, we don't want to push wrong content with wrong matching conditions.

Let's look at this following example:

{
    "_id" : {
      "$oid": "64d4dfbe0e349e259d611b7b"
    },
    "people" : [
        {
            "name" : "lili",
            "loves" : "mario party"
        },
        {
            "name" : "miaozi",
            "loves" : "fire emblem"
        },
        {
            "name" : "kai",
            "loves" : "hollow knight"
        }
    ]
}

when we update it with the following query and update:

// query
{
  _id : ObjectId("64d4dfbe0e349e259d611b7b"),
  people: {$not: {$elemMatch: {name: 'not a thing'}}}
}

// update
{
  $push : {
    people: {
      name: 'kai',
      loves: 'cs'
    }
  }
}

will result in

{
    "_id" : ObjectId("64d4dfbe0e349e259d611b7b"),
    "people" : [
        {
            "name" : "lili",
            "loves" : "mario party"
        },
        {
            "name" : "miaozi",
            "loves" : "fire emblem"
        },
        {
            "name" : "kai",
            "loves" : "hollow knight"
        },
        {
            "name" : "kai",
            "loves" : "cs"
        }
    ]
}

we matched with not a thing but anyway pushed the object kai in there.

bearzk/mongodb-idempotent-array-push.md

Idempotent array push