Why MongoDB Duplicate Documents Has even after adding unique Key

In this post, we will learn How to avoid MongoDB Duplicate Documents even after adding a unique key the reasons for this thing could be different which we had explained here one by one.


Reasons for Duplicate

There could be a number of reasons for the same where even after defining the unique key we are getting multiple values and we had listed all the possible reasons here.

Already Exist

There might be a change that there already exist multiple values before making it a unique value as it will not be removing the records that are already stored in the collection and after that, we declared it as unique.

But we can always remove them by a simple query given below the collections all will be unique after it.

  { $group: { _id: { fieldToCheck: "$fieldToCheck" }, count: { $sum: 1 } } },
  { $match: { count: { $gt: 1 } } }


The query above will group the document by field to check the field and return those groups which have the count as greater than 1 (which is nothing but multiple values) and now we can always delete them as per our choice whichever we want.

A Unique Key s Not Enforced

There might be a situation when the field we apply a unique key filter may not full fill the requirements as it might not be indexed so first we need to create indexes and then again apply the unique key filter for the same we have given command below.

db.collection.createIndex({ uniqueField: 1 }, { unique: true })

Here it will simply create the indexes and make the field unique.

Different Unique Key

There might be a situation there are multiple unique keys so we might apply it to different fields.

Different Collection

At last, we need to keep certain things in mind like making sure that you are inserting the documents into the correct collection. If you are accidentally inserting documents into a different collection than the one with the unique key, duplicates may still occur which might have different purposes which might not be listed here to avoid such a situation we can again apply the same unique key to the field.



To learn more about How to avoid MongoDB Duplicate Documents even add a unique key visit: by stack overflow

To learn more about MongoDB and tutorials related to it visit: MongoDB Problems And Tutorials

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