how to find duplicate field in subdocument array in MongoDB -

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i'm trying find way find , remove duplicate object same field in subdocument array.

basically collection has schema.

[{   "_id": objectid("578953db976cea724bbdbb60"),   "units": [{     "description": "a1"   }, {     "description": "b1"   }, {     "description": "a1"   }, {     "description": "c1"   }] }, {   "_id": objectid("578953db976cea724bbdbb61"),   "units": [{     "description": "a2"   }, {     "description": "b2"   }, {     "description": "a2"   }, {     "description": "c2"   }] }]

i wish find , remove duplicate field same description.

so first, need identify "a1" , a2" respectively.

thanks.

update (output should be):

[{    "_id": objectid("578953db976cea724bbdbb60"),    "units": [{      "description": "a1"    }, {      "description": "b1"    }, {      "description": "c1"    }]  }, {    "_id": objectid("578953db976cea724bbdbb61"),    "units": [{      "description": "a2"    }, {      "description": "b2"    }, {      "description": "c2"    }]  }]

use javascript

db.collection.find().foreach(function(element){    var descriptions = element.units.map(function(unit){return unit.description;})    element.units = element.units.filter(function(value, index, array){        return descriptions.indexof(value) == index;    })    db.save(element); })

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