MongoDB: Check Array Field for Unique or Array Value

When working with MongoDB, it's common to have arrays as fields in your documents. Sometimes, you may need to check if a value or an entire array in a field is unique.

Índice
  1. Checking for Unique Value in Array Field
  2. Checking for Unique Array in Field

Checking for Unique Value in Array Field

To check if a value is unique in an array field, you can use the $addToSet operator along with the $each modifier.


db.collection.update(
    {_id: docId},
    {$addToSet: {arrayField: {$each: [valueToCheck]}}}
)

If the value is already in the array, nothing will be added and the update operation will return a success message. If the value is not in the array, it will be added.

Checking for Unique Array in Field

If you need to check if an entire array in a field is unique, there's a bit more work involved. First, you'll need to use the $group aggregation stage to group documents by the array field.


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

This will return all documents where the array field is not unique. You can then use the $lookup stage to join the original collection with the result of the $group stage, and use the $unset operator to remove any documents where the array field is not unique.


db.collection.aggregate([
    {$group: {
       _id: "$arrayField",
       count: {$sum: 1}
    }},
    {$match: {
       count: {"$gt": 1}
    }},
    {$lookup: {
       from: "collection",
       localField: "_id",
       foreignField: "arrayField",
       as: "duplicates"
    }},
    {$unwind: "$duplicates"},
    {$unset: "duplicates"}
])

With these techniques, you can efficiently check for unique values and arrays in MongoDB array fields.

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