How to delete an array element that has an index in mongoDB?

Question

The document looks like this,

{
"_id":"ffffff999999999f9ff9f9f9f",
 "Name" : "John Doe",
  "Array" : [{
      "Id1" : "a8ed3d86b8464e0cae4672cef3862860",
      "Id2" : "6d7aac14b1e142abafde167d928e3dbc",
      "Id3" : "2323232"
    }]
}

There is a uniqueness index on the combination of the 3 array elements. So the index looks like this,

        "key" : {
                "Array.Id1" : 1,
                "Array.Id2" : 1,
                "Array.Id3" : 1
        },
        "unique" : true,

My requirement: I would like to find documents based on "_id" value and remove the array element completely.

Problem: I was trying something like this,

db.Collection.update({"_id":"ffffffff52a8a15ce4b05d6a8d40f973"},{$unset:{Array:1}})

When i try to unset the array element, the first update goes through but the consecutive ones fail with the following error,

*E11000 duplicate key error index: int.Collection.$Array.Id1_1_Array.Id2_1_Array.Id3_1 dup key: { : null, : null, : null }*

I am wondering if there is any work around for this issue.

I would have to run this on a huge collection and modifying the indexes is not an option.

Any suggestions would be greatly helpful.

Thanks.

You'll need to modify or remove the index.

WiredPrairie
– WiredPrairie

2013-12-11 19:39:45 +00:00
Commented Dec 11, 2013 at 19:39 — WiredPrairie
– WiredPrairie, Commented Dec 11, 2013 at 19:39

4J41 · Accepted Answer · 2013-12-11 19:46:38Z

1

Unique indexes work that way. You cannot have duplicates on the indexed fields even null values(or leave it undefined). However the sparse option allows you to achieve this.

Usage: db.collection.ensureIndex( { a: 1 }, { unique: true, sparse: true } )

So if you are going to unset the array, you might want to re-index with the sparse option.

Warning: Using these indexes will sometimes result in incomplete results 
when filtering or sorting results, because sparse indexes are not complete 
for all documents in a collection.

edited Dec 11, 2013 at 19:46

answered Dec 11, 2013 at 18:57

4J41

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1 Comment

user1840125 Over a year ago

Thanks for your help. since re-indexing is not an option for me. i would try to set the value to something else(some dummy unique value) during the update.

chk · Accepted Answer · 2013-12-11 18:45:51Z

As demonstrated in this minimal example, it is not possible to have a unique-indexed field unset more than once right from the start - the problem not only occurs when you try to unset it later on.

> db.coll.ensureIndex({indexField:1}, {unique: true})
> db.coll.insert({name: "doc without index field 1"})
> db.coll.insert({name: "doc without index field 2"})
E11000 duplicate key error index: test.coll.$indexField_1  dup key: { : null }

You will have to replace the field with something else - not necessarily an array, but it will have to be unique...

You can use sparse as shown in the other answer to allow unset values.

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