How to remove “attribute” on array in embedded document in MongoDb?

Question

I need to remove every attribute called "cast_id" from this document:

{
  "_id": ObjectId("5b895f15674e2ddf78bcc6f8"),
  "cast": [
    {
      'cast_id': 14,
      'character': 'Woody (voice)',
      'credit_id': '52fe4284c3a36847f8024f95',
      'gender': 2,
      'id': 31,
      'name': 'Tom Hanks',
      'order': 0,
      'profile_path': '/pQFoyx7rp09CJTAb932F2g8Nlho.jpg'
    },
    {
      'cast_id': 15,
      'character': 'Buzz Lightyear (voice)',
      'credit_id': '52fe4284c3a36847f8024f99',
      'gender': 2,
      'id': 12898,
      'name': 'Tim Allen',
      'order': 1,
      'profile_path': '/uX2xVf6pMmPepxnvFWyBtjexzgY.jpg'
    },
    {
      'cast_id': 16,
      'character': 'Mr. Potato Head (voice)',
      'credit_id': '52fe4284c3a36847f8024f9d',
      'gender': 2,
      'id': 7167,
      'name': 'Don Rickles',
      'order': 2,
      'profile_path': '/h5BcaDMPRVLHLDzbQavec4xfSdt.jpg'
    },
    {
      'cast_id': 17,
      'character': 'Slinky Dog (voice)',
      'credit_id': '52fe4284c3a36847f8024fa1',
      'gender': 2,
      'id': 12899,
      'name': 'Jim Varney',
      'order': 3,
      'profile_path': '/eIo2jVVXYgjDtaHoF19Ll9vtW7h.jpg'
    },
    {
      'cast_id': 18,
      'character': 'Rex (voice)',
      'credit_id': '52fe4284c3a36847f8024fa5',
      'gender': 2,
      'id': 12900,
      'name': 'Wallace Shawn',
      'order': 4,
      'profile_path': '/oGE6JqPP2xH4tNORKNqxbNPYi7u.jpg'
    },
    {
      'cast_id': 19,
      'character': 'Hamm (voice)',
      'credit_id': '52fe4284c3a36847f8024fa9',
      'gender': 2,
      'id': 7907,
      'name': 'John Ratzenberger',
      'order': 5,
      'profile_path': '/yGechiKWL6TJDfVE2KPSJYqdMsY.jpg'
    },
    {
      'cast_id': 20,
      'character': 'Bo Peep (voice)',
      'credit_id': '52fe4284c3a36847f8024fad',
      'gender': 1,
      'id': 8873,
      'name': 'Annie Potts',
      'order': 6,
      'profile_path': '/eryXT84RL41jHSJcMy4kS3u9y6w.jpg'
    },
    {
      'cast_id': 26,
      'character': 'Andy (voice)',
      'credit_id': '52fe4284c3a36847f8024fc1',
      'gender': 0,
      'id': 1116442,
      'name': 'John Morris',
      'order': 7,
      'profile_path': '/vYGyvK4LzeaUCoNSHtsuqJUY15M.jpg'
    },
    {
      'cast_id': 22,
      'character': 'Sid (voice)',
      'credit_id': '52fe4284c3a36847f8024fb1',
      'gender': 2,
      'id': 12901,
      'name': 'Erik von Detten',
      'order': 8,
      'profile_path': '/twnF1ZaJ1FUNUuo6xLXwcxjayBE.jpg'
    },
    {
      'cast_id': 23,
      'character': 'Mrs. Davis (voice)',
      'credit_id': '52fe4284c3a36847f8024fb5',
      'gender': 1,
      'id': 12133,
      'name': 'Laurie Metcalf',
      'order': 9,
      'profile_path': '/unMMIT60eoBM2sN2nyR7EZ2BvvD.jpg'
    },
    {
      'cast_id': 24,
      'character': 'Sergeant (voice)',
      'credit_id': '52fe4284c3a36847f8024fb9',
      'gender': 2,
      'id': 8655,
      'name': 'R. Lee Ermey',
      'order': 10,
      'profile_path': '/r8GBqFBjypLUP9VVqDqfZ7wYbSs.jpg'
    },
    {
      'cast_id': 25,
      'character': 'Hannah (voice)',
      'credit_id': '52fe4284c3a36847f8024fbd',
      'gender': 1,
      'id': 12903,
      'name': 'Sarah Freeman',
      'order': 11,
      'profile_path': None
    },
    {
      'cast_id': 27,
      'character': 'TV Announcer (voice)',
      'credit_id': '52fe4284c3a36847f8024fc5',
      'gender': 2,
      'id': 37221,
      'name': 'Penn Jillette',
      'order': 12,
      'profile_path': '/zmAaXUdx12NRsssgHbk1T31j2x9.jpg'
    }
  ]
}

This document contains at least 45000 item like that, and I have to remove from each one, the "attribute" "cast_id" and more "attribute", like "gender" or "profile_path" .

I try to use $pull , $pullAll , $unset , and remove in this way, but unsuccessfully. ( "crediti" is the name of my collection)

db.crediti.update({}, {$pull: {cast: {cast_id}}}, {multi:true})
db.crediti.update({}, {$unset: {cast: {cast_id}}}, {multi:true})
db.crediti.update({}, {$pullAll: {cast: {cast_id}}}, {multi:true})
db.crediti.remove({"cast.cast_id"})

Otherwise, I try to use $unset in this way:

db.crediti.update({}, {$unset:{"cast.cast_id":1}},{multi:true})

And the result is:

WriteResult({ "nMatched" : 45476, "nUpserted" : 0, "nModified" : 0 })

They matched, but weren't modified.

The result that I want, is to maintain other attribute, like this:

{
    "_id" : ObjectId("5b895f15674e2ddf78bcc6f8"),
    "cast" : "[{'character': 'Woody (voice)', 'id': 31, 'name': 'Tom Hanks'}, etc...

How can I do this?

Answer 1

we can do this using $unset and filtered positional operator

the update to remove all the cast_id from the array is shown below

db.crediti.update( 
   {}, 
   {$unset:{"cast.$[].cast_id":""}},
   {multi:1}
);

Update query has three parts,

The first part is to pick all the documents in the collection which we are going to update, here we have given it as empty document so it applies to all documents in the collection.

The second part of the query is using filtered positional operator together with $unset, which does the job of removing the cast_id from all documents inside the array.

The third part of the query does the update for multiple documents on the collection

Please note that $[identifier] works from version 3.6