Remove a field from all elements in array in mongodb

Sorry to disappoint you, but your answer

db.coll.update({
   _id:235399,
   "casts.crew.withBase": {$exists: true}
},{
   $unset: {
      "casts.crew.$.withBase" : true
   }
},false,true)

is not correct. Actually it will remove the value, BUT only from the first occurrence of the subdocument, because of the way positional operator works:

the positional $ operator acts as a placeholder for the first element that matches the query document

You also can not use $unset (as you tried before) because it can not work on arrays (and are you basically trying to remove a key from a document from the array). You also can not remove it with $pull, because pull removes all the array, not just a field of it.

Therefore as far as I know you can not do this with a simple operator. So the last resort is doing $find and then forEach with save. You can see how to do this in my answer here. In your case you need to have another loop in forEach function to iterate through array and to delete a key. I hope that you will be able to modify it. If no, I will try to help you.

P.S. If someone looks a way to do this - here is Sandra's function

db.coll.find({_id:235399}).forEach( function(doc) {
  var arr = doc.casts.crew;
  var length = arr.length;
  for (var i = 0; i < length; i++) {
    delete arr[i]["withBase"];
  }
  db.coll.save(doc);
});

I found a way to unset this lists without having to pull up the object (meaning, just doing an update), it's pretty hackish but if you have a huge database it will make the deal:

db.coll.update({},{$unset: {"casts.crew.0.withBase" : 1, "casts.crew.1.withBase" : 1} }, {multi: 1})

In other words, you have to calculate how many objects there can be in any of your documents list and add those numbers explicitly, in this case as {casts.crew.NUMBER.withBase: 1}.

Also, to count the longest array in a mongodb object, an aggregate can be done, something like this:

db.coll.aggregate( [   { $unwind : "$casts.crew" },   { $group : { _id : "$_id", len : { $sum : 1 } } },   { $sort : { len : -1 } },   { $limit : 1 } ], {allowDiskUse: true} )

Just want to emphasize that this is not a pretty solution but is way faster than fetching and saving.

You can use the new positional identifier to update multiple elements in array in 3.6.

Something like

 db.coll.update( {_id:235399}, {$unset: {"casts.crew.$[].withBase":""}} )

$[] removes all the withBase property from the crews array. It acts as a placeholder for updating all elements in array.

Use multi true to affect multiple documents.