Find all duplicate documents in a MongoDB collection by a key field

Question

Suppose I have a collection with some set of documents. something like this.

{ \"_id\" : ObjectId(\"4f127fa55e7242718200002d\"), \"id\":1, \"name\" : \"foo\"         


        

Answer 1

aggregation pipeline framework can be used to easily identify documents with duplicate key values:

// Desired unique index: 
// db.collection.ensureIndex({ firstField: 1, secondField: 1 }, { unique: true})

db.collection.aggregate([
  { $group: { 
    _id: { firstField: "$firstField", secondField: "$secondField" }, 
    uniqueIds: { $addToSet: "$_id" },
    count: { $sum: 1 } 
  }}, 
  { $match: { 
    count: { $gt: 1 } 
  }}
])

~ Ref: useful information on an official mongo lab blog:

https://blog.mlab.com/2014/03/finding-duplicate-keys-with-the-mongodb-aggregation-framework