Locate identical values within an array in MongoDB, even if they exist independently of an object

{
    "_id" : ObjectId("15672"),
    "userName" : "4567",
    "library" : [ 
        {
            "serialNumber" : "Book_1"
        }, 
        {
            "serialNumber" : "Book_2"
        }, 
        {
            "serialNumber" : "Book_4"
        }
    ]
},
{
    "_id" : ObjectId("123456"),
    "userName" : "123",
    "library" : [ 
        {
            "serialNumber" : "Book_2"
        }
    ]
},
{
    "_id" : ObjectId("1835242"),
    "userName" : "13526",
    "library" : [ 
        {
            "serialNumber" : "Book_7"
        }, 
        {
            "serialNumber" : "Book_6"
        }, 
        {
            "serialNumber" : "Book_5"
        }, 
        {
            "serialNumber" : "Book_4"
        }, 
        {
            "serialNumber" : "Book_3"
        }, 
        {
            "serialNumber" : "Book_5"
        }
    ]
}

I need assistance with a query that can identify usernames where duplicate serial numbers exist across libraries, excluding one specific username's library from the comparison.

javascriptarraysmongodbmongodb-queryaggregation-framework

Answer №1

Give this query a shot:

db.collection.aggregate([
    /** You can optionally include the first match stage if all your documents are arrays and not empty */
    { $match: { $expr: { $and: [{ $eq: [{ $type: "$library" }, "array"] }, { $ne: ["$library", []] }] } } },
    /** Create a new field 'allUnique' in each document, which will be false if there are duplicates within the 'library' elements */
    {
        $addFields: {
            allUnique: {
                $eq: [
                    {
                        $size:
                        {
                            $reduce: {
                                input: "$library.serialNumber",
                                initialValue: [], // start with an empty array
                                /** Iterate over the serialNumber's array from library and add current value only if it's unique, resulting in an array of unique values */
                                in: { $cond: [{ $in: ["$$this", "$$value"] }, [], { $concatArrays: [["$$this"], "$$value"] }] }
                            }
                        }
                    },
                    {
                        $size: "$library"
                    }
                ]
            }
        }
    },
    /** Get documents where allUnique is false */
    {
        $match: {
            allUnique: false
        }
    },
    /** Project only required fields and remove the default _id projection */
    {
        $project: {
            userName: 1,
            _id: 0
        }
    }
])

Another approach could involve using $unwind, but this may not be ideal for large datasets as it can significantly increase the size of your collection.

Test it out: MongoDB-Playground

You can also refer to @Dennis's answer in this thread on removing duplicate entries from an array for another method:

db.collection.aggregate([
  {
    $match: {
      $expr: {
        $and: [
          {
            $eq: [
              {
                $type: "$library"
              },
              "array"
            ]
          },
          {
            $ne: [
              "$library",
              []
            ]
          }
        ]
      }
    }
  },
  {
    $addFields: {
      allUnique: {
        $eq: [
          {
            $size: {
              "$setUnion": [
                "$library.serialNumber",
                []
              ]
            }
          },
          {
            $size: "$library"
          }
        ]
      }
    }
  },
  {
    $match: {
      allUnique: false
    }
  },
  {
    $project: {
      userName: 1,
      _id: 0
    }
  }
])

Test it here: MongoDB-Playground

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