AddToSet new object or update counter in existings - mongodb

Question

I have document {id:1, data: [{'name': 'Bob', 'counter':1}, {'name':'Jack', 'counter':1}]}
What I'm expecting:
query:
db.inventory.update(
{ _id: 1 },
{ $addToSet: { data: { $each: [{'name': 'Bob'}, {'name':'Jack'}, {'name':'John'}] } } }
)
result:
{id:1, data: [{'name': 'Bob', 'counter':2}, {'name':'Jack', 'counter':2}, {'name':'John', 'counter':1}]}

Answer 1

You won't be able to do this in a single query. The $addToSet operator will only work if the element being added and the element that exists are an exact match. Instead, you'll need to do it in multiple parts:
// Insert element for Bob if it doesn't exist.
db.inventory.update(
{
_id: 1,
"data.name": {$ne: "Bob"}
},
{
$push: {
data: {
name: "Bob",
counter: 0 // Initialized to 0 so that the first increment results in the expected value of 1.
}
}
}
)
// Increment the counter for Bob.
db.inventory.update(
{
_id: 1,
"data.name": "Bob"
},
{
$inc: {
"data.$.counter": 1
}
}
)
// Repeat as necessary for each element you wish to insert.
This is simply a limitation that you need to work around with your existing document structure. If you modify your document structure such that data is a nested sub-document with each name being a field within that sub-document, you could make this work with a single query:
// Version 1: flat value.
db.inventory.update(
{ _id: 1 },
{ $inc: {
"data.Bob": 1,
"data.Jack": 1
}}
)
/*
Document will look like this:
{
_id: 1,
data: {
Bob: 2,
Jack: 2,
John: 1
}
}
*/
// Version 2: nested sub-document.
db.inventory.update(
{ _id: 1 },
{ $inc: {
"data.Bob.counter": 1,
"data.Jack.counter": 1
}}
)
/*
Document will look like this:
{
_id: 1,
data: {
Bob: {
counter: 2
},
Jack: {
counter: 2
},
John: {
counter: 1
}
}
}
*/
Be warned, however, that you will not be able to index data effectively if you go with this solution, so querying efficiently on e.g. all documents containing elements with data.$.counter > 1 simply will not be possible.
The trade-offs are yours to consider. You can either have efficient updates or efficient lookups, but having both is unlikely to happen. I would personally recommend updating each element individually, but you will know your program's needs far better than I will.