Mongo is it possible to increment array element if not present - arrays

Question

Hi I am trying to increment an element inside an array as below
{ "_id" : 1
"myarray": [
{"a": 1}
{"b": 1}
{"c": 1}
]
}
{ "_id" : 2
"myarray": [
{"a": 4}
{"b": 7}
{"c": 9}
]
}
db.mycollection({"_id": 1}, {"$inc": {"myarray.$.a"}});
db.mycollection({"_id": 2}, {"$inc": {"myarray.$.b"}});
I am aware of the mongo $ operator for identifying an element in array as used above.
The problem I am tackling is in some scenarios of my data does not have the array
{ "_id" : 3
}
{ "_id" : 4
}
and if I execute db.mycollection({"_id": 3}, {"$inc": {"myarray.$.a"}}); It does not update the document with id since myarray does not exist the doc.
so the immediate fix I made was to initialize myarray to be zero whenever I insert a doc
{ "_id" : 3
"myarray": [
{"a": 0}
{"b": 0}
{"c": 0}
]
}
{ "_id" : 4
"myarray": [
{"a": 0}
{"b": 0}
{"c": 0}
]
}
Is there a way to directly update with pre-initializing the array element with zero?

Answer 1

db.mycollection({"_id": 1}, {"$inc": {"myarray.$.a"}}); is wrong in so many ways:
First of all it is db.mycollection.update or updateOne, updateMany etc.
Next, $inc requires a number on how much you want to increment. If it is 1, you have to specify it explicitly: {"$inc": {"myarray.$.a": 1}}
Finally, the $ operator you are aware of requires a match from the query part (the first argument of the update* functions). Please read the page you are referring to in your question.
To answer the question - no, there is no such way. It's not about the $inc - it actually initialises a field with 0 if it doesn't exists. It's $ that doesn't push a new element into array because the document won't pass the filter stage where you specify which matching subdocument should $ refer to.