How to remove duplicate values inside a list in mongodb

Question

I have a mongodb collection . When I do.

db.bill.find({})

I get,

Answer 1

MongoDB 4.2 collection updateMany method's update parameter can also be an aggregation pipeline (instead of a document). The pipeline supports $set, $unset and $replaceWith stages. Using the $setIntersection aggregation pipeline operator with the $set stage, you can remove the duplicates from an array field and update the collection in a single operation.

An example:

arrays collection:

{ "_id" : 0, "a" : [ 3, 5, 5, 3 ] }
{ "_id" : 1, "a" : [ 1, 2, 3, 2, 4 ] }

From the mongo shell:

db.arrays.updateMany(
   {  },
   [
      { $set: { a: { $setIntersection: [ "$a", "$a" ] } } }
   ]
)

The updated arrays collection:

{ "_id" : 0, "a" : [ 3, 5 ] }
{ "_id" : 1, "a" : [ 1, 2, 3, 4 ] }

The other update methods, update(), updateOne() and findAndModify() also has this feature.