Currently mongoimport's merge mode will $set the entire document from a CSV. This means that it overwrites subdocuments that already exist. It might be useful to merge subdocuments. For example, this CSV:

_id,a.b,a.c
1,2,3

With this document in the database:

{
    _id: 1,
    a: {
        d: 4
    }
} 

currently results in:

{
    _id: 1,
    a: {
        b: 2,
        c: 3
    }
} 

while we might want:

{
    _id: 1,
    a: {
        b: 2,
        c: 3,
        d: 4
    }
} 

We can do the following to set subdocument fields:

{$set: {"a.b": 2}}

but we will need separate handling for arrays. Merging arrays would also cause an issue with ordering since the CSV specifies the exact position of elements in the array.