How to move array elements between collections?

Question

I have the following collection with three documents:

{
   _id: 1
   items: ['1a', '1b', '1c']
},
{
   _id: 2,
   items: ['2a', '2b', '2c']
},
{
   _id: 3,
   items: []
}

I have a query that have to move '1a','1c','2a','2b' elements from their corresponding documents into documents with _id: 3.

I need to get the following result:

{
   _id: 1
   items: ['1b']
},
{
   _id: 2,
   items: ['2c']
},
{
   _id: 3,
   items: ['1a','1c', '2a', '2b']
}

Help me please. What is better solution for resolving my problem?

Answer 1

I don't think of any specific operators for move operation. But, I think you could use $push or $addToSet operator for this add operation like below and then remove those items from 1 & 2 documents.

db.testcollection.update({_id:3}, {$addToSet:{ ... }})

Answer 2

firstly, you need to explain why you want to do this. Because what you want to do is just this operator without adding other operation, i wonder the simplest way is do like this:

1. get the result from db
2. do the operation what you want
3. delete the old record and save the modify record into db

although the idea sounds like fool, but it's usful. if you have deep requirement, please give them to us.

Answer 3

use update() with $set operation

db.test1.update({_id:1},{$set:{items:['1b']}})

this will update your array set.

Thanks,