MongoDB - Split a Document with an Array of Subdocuments into 2 documents, with half the Subdocuments in each

Question

Good day all,

I have a set of documents which are hitting the BSON size limit due to the number of items in an array of Subdocuments. I'm trying to apply a new field to these subdocuments as part of a schema update but it's failing on these specific documents because the new field pushes the BSON size limit over.

What I want to do is effectively split the Document(s) in 'half' based on the array of subdocuments, so I would have Document1 (the original) with the array now containing (for example) subdocuments 1-2, and Document1B having the array containing 3-4.

Here is an example of the basic layout

{ 
    "_id" : ObjectId("5f11d4c28663f32e940696e0"), 
    "DocumentId" : "0000001", 
    "Items" : [
        {
            "ItemId" : NumberInt(1), 
            "ItemType" : NumberInt(1), 
            "ClassType" : "1", 
            "Created" : ISODate("2012-03-13T11:13:22.000+0000"), 
            "LastUpdated" : ISODate("2020-03-12T15:52:39.000+0000") 
        }, 
        {
            "ItemId" : NumberInt(2), 
            "ItemType" : NumberInt(1), 
            "ClassType" : "1", 
            "Created" : ISODate("2012-03-13T11:13:22.000+0000"), 
            "LastUpdated" : ISODate("2020-03-12T15:52:39.000+0000") 
        }, 
        {
            "ItemId" : NumberInt(3), 
            "ItemType" : NumberInt(1), 
            "ClassType" : "1", 
            "Created" : ISODate("2012-03-13T11:13:22.000+0000"), 
            "LastUpdated" : ISODate("2020-03-12T15:52:39.000+0000") 
        }, 
        {
            "ItemId" : NumberInt(4), 
            "ItemType" : NumberInt(1), 
            "ClassType" : "1", 
            "Created" : ISODate("2012-03-13T11:13:22.000+0000"), 
            "LastUpdated" : ISODate("2020-03-12T15:52:39.000+0000") 
        }, 
    ], 
    "Priority" : NumberInt(0), 
    "Status" : NumberInt(3), 
    "FileChecksum" : "ca1fbd9c8e1669bcf4155657d3600fa4", 
    "CreatedBy" : "[email protected]", 
    "LastUpdatedBy" : null, 
    "AssignedBy" : "[email protected]", 
    "DeletedBy" : null, 
    "LastUpdated" : ISODate("2020-03-12T15:52:39.000+0000"), 
    "Created" : ISODate("2012-03-13T08:13:23.000+0000"), 
    "Assigned" : ISODate("2012-03-13T11:10:48.000+0000"), 
    "Completed" : ISODate("2012-03-13T11:13:22.000+0000"), 
    "DeletedDate" : null, 
    "AssignedTo" : "[email protected]", 
    "RejectionReason" : null, 
    "Deleted" : false, 
    "DeletionReason" : null
}