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

MongoDB Duplicate Documents Despite Unique Key

In MongoDB, creating a unique index on multiple fields should prevent duplicate documents with the same values for those fields. However, in some cases, duplicate documents may still be inserted even after adding an index.

Problem:

A unique index was created on the "uid" and "sid" fields in a collection, but documents with duplicate values for these fields were still being inserted using the PHP driver.

Solution:

MongoDB Shell

1. Check for Existing Duplicates:

   • Execute the following query to identify existing duplicate documents:

   db.user_services.aggregate([
       { "$group": {
           "_id": { "uid": "$uid", "sid": "$sid" },
           "dups": { "$push": "$_id" },
           "count": { "$sum": 1 }
       }},
       { "$match": { "count": { "$gt": 1 } }}
   ])

2. Remove Duplicates:

   • For each document identified as a duplicate, remove all but the first (original) document:

   db.user_services.remove({ "_id": {"$in": doc.dups }});

3. Create Unique Index:

   • Once all duplicates have been removed, create the unique index using the following command:

   db.user_services.createIndex({"uid":1 , "sid": 1},{unique:true})

PHP

1. Use Update with Upsert:

   • Instead of using .insert(), switch to using 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>

2. Handle Multiple Updates:

   • Refactor the update query to handle multiple update operations correctly:

   <code class="php">$collection->update(
       array( "uid" => 1, "sid" => 1 ),
       array(
           '$set' => array( "field" => "this" ),
           '$inc' => array( "counter" => 1 ),
           '$setOnInsert' => array( "newField" => "another" )
       ),
       array( "upsert" => true )
   );</code>

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