Why Am I Still Seeing Duplicate Documents in MongoDB Despite Having a Unique Index?

MongoDB Duplicate Documents Despite Unique Key

When creating a collection in MongoDB and adding a unique index using createIndex({"uid":1 , "sid": 1},{unique:true,dropDups: true}), it may appear that duplicate documents are still being inserted despite the unique key constraint.

MongoDB Shell Resolution

Upon examining the index creation, an error message indicating "duplicate key error" is likely encountered. This is because MongoDB 3.0.0 had a bug that prevented the index from being created when duplicates existed.

To resolve this, first remove duplicate records using an aggregate query and the $match operator.

<code class="javascript">db.events.aggregate([
    { "$group": {
        "_id": { "uid": "$uid", "sid": "$sid" },
        "dups": { "$push": "$_id" },
        "count": { "$sum": 1 }
    }},
    { "$match": { "count": { "$gt": 1 } }}
]).forEach(function(doc) {
    doc.dups.shift();
    db.events.remove({ "_id": {"$in": doc.dups }});
});</code>

Once duplicates are removed, the unique index can be created without encountering an error.

PHP Solution

To insert documents that don't already exist or update existing documents, use the update() method with the upsert option set to true.

<code class="php">$collection->update(
    array( "uid" => 1, "sid" => 1 ),
    array( '$set' => $someData ),
    array( 'upsert' => true )
);</code>

This will insert the document if it does not exist, otherwise it will update the document by setting the fields specified in the $set array.

Additional Considerations

• The dropDups option used in the initial index creation is no longer available in MongoDB 3.0.0.
• The .update() method syntax requires three arguments: "query", "update", and "options".
• Update operations within the "update" document should not access the same path more than once.

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