$unset not working on array of objects?

Question

I have a collection with documents like below:

{
   title: "whatever",
   answers: [
      {id: 10, question: ObjectId("54380350a52147000aad4e9b")},
      {id: 13, question: ObjectId("54380350a52147000aad4e9c")},
      {id: 33}
   ]
}

I'm have attempted to unset the question attribute using the commands below:

db.participants.update({}, {$unset: {"answers.question": ""}}, {upsert: false, multi:true} )

and

db.participants.update({}, {$unset: {"answers.question": 1}}, {upsert: false, multi:true} )

Both of these spit out: WriteResult({ "nMatched" : 628795, "nUpserted" : 0, "nModified" : 0 }) when completed, but no documents are updated (as the message suggests). Any idea why my update isn't working?

Gergo Erdosi · Accepted Answer · 2014-10-13 21:20:58Z

2

Your update is not working because answers is an array. So you should use the positional operator:

db.participants.update({ 'answers.question': { $exists: true } }, { $unset: { 'answers.$.question': '' } }, { upsert: false, multi: true });

However it updates only one item in the array. This is a limitation of MongoDB, see SERVER-1243.

You can use a forEach to iterate through the items of the array:

db.eval(function () {

    db.participants.find({ 'answers.question': { $exists: true } }).forEach(function (participant) {

        participant.answers.forEach(function (answer) {

            delete answer.question;
        });

        db.participants.update({ _id: participant._id }, { $set: { answers: participant.answers } });
    });
});

answered Oct 13, 2014 at 21:20

Gergo Erdosi

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3 Comments

Abe Miessler Over a year ago

+1, thanks for explaining why it wasn't working. Ended up using a modified version of your first update that matched on a condition that would return each array element and then used the positional operator. Seemed a bit simpler than the forEach solution.

Gergo Erdosi Over a year ago

Your query with the positional operator still updates only one item in the array. The condition doesn't affect the update in that case.

Abe Miessler Over a year ago

Oh you're right. Bummer, that seems pretty complex to clear out a value.

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