MongoDB - replace item in nested array

Question

MongoDB does not allow to replace an item in an array in a single operation. Instead it's a pull followed by a push operation.

Unfortunately we have a case where we end up with a race condition on the same item in the array on parallel requests (distributed environment), i.e. 2x pull runs first, then 2x push. This results in duplicate entries, e.g.

{
    "_id": ...,
    "nestedArray": [
        {
            "subId": "1"
        },
        {
            "subId": "1"
        },
        {
            "subId": "2"
        }
    ]
}

Are there any workarounds?

enrichz · Accepted Answer · 2017-04-11 07:34:28Z

1

I usually use an optimistic lock for this situation. To prepare for this, you need to add a version field to your model, which you will increment each time you modify that model. Then you use this method:

Model.findOneAndUpdate(
        {$and: [{_id: <current_id>}, {version: <current_version>}]}, 
        {nestedArray: <new_nested_array>})
  .exec(function(err, result) {
    if(err) {
      // handle error
    }
    if(!result) {
      // the model has been updated in the mean time
    }
    // all is good
});

This means that you first need to get the model and compute the new array <new_nested_array>. This way you can be sure that only one modification will take place for a certain version. Hope I explained myself.

answered Apr 11, 2017 at 7:34

enrichz

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2 Comments

Benjamin E. Over a year ago

I guess that's why MongoDB still gets hate from the SQL camp... :) Came up with such a solution before but it felt not right. Thanks

enrichz Over a year ago

You need to change your whole approach to work with MongoDB. I don't know your requirements, but maybe try to investigate Redis, which can be used in conjunction with MongoDB and supports transactions.

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