query for number of array elements containing some field in mongodb - arrays

Question

I have a collection of documents that look like this:
{
my_array : [
{
name : "...",
flag : 1 // optional
},
...
]
}
How can I query for documents that exactly one of their elements contain the flag field?
This will return documents where my_array length is exactly 1:
db.col.find({ "my_array" : { $size : 1 } })
This will return documents where at least one of my_array's object contain the flag field:
db.col.find({ "my_array.flag" : { $exists : true } })
And this will look for documents where flag is an array of size one:
db.col.find({ "my_array.flag" : { $size : 1 } })
I want to somehow combine the first two queries, checking for some inner field existence, and then querying for the size of the filtered array

Answer 1

You can try $redact with $filter for your query.
$filter with $ifNull to keep the matching element followed by $size and $redact and compare result with 1 to keep and else remove the document.
db.col.aggregate(
[{
$match: {
"my_array.flag": {
$exists: true
}
}
}, {
$redact: {
$cond: {
if: {
$eq: [{
$size: {
$filter: {
input: "$my_array",
as: "array",
cond: {
$eq: [{
$ifNull: ["$$array.flag", null]
}, "$$array.flag"]
}
}
}
}, 1]
},
then: "$$KEEP",
else: "$$PRUNE"
}
}
}]
)

Answer 2

Use Aggregation framework in mongodb,
pipeline stage 1: Match ,
checking for some inner field existence,
{ "my_array.flag" : { $exists : true } }
pipleline stage 2: Project,
querying for the size of the filtered array,
{$project:{ "my_array":{$size:"$my_array"}}}
mongoshell Command ,
db.col.aggregate([{
$match: {
"my_array.flag": {
$exists: true
}
}
}, {
$project: {
"my_array": {
$size: "$my_array"
}
}
}])

Answer 3

Following the current answers about using aggregate, here's a working solution using aggregation:
db.col.aggregate([
{ $match : { "my_array.flag" : { $exists : true } } },
{ $unwind : "$my_array" },
{ $match : { "my_array.flag" : { $exists : true } } },
{ $group : { _id : "$_id", count_flags : { $sum : 1 } } },
{ $match : { "count_flags" : 1 } }
])
Here are the steps explained:
Filter to only documents that contain at least one flag field (this will use my index to dramatically reduce the number of documents I need to process)
Unwind the my_array array, to be able to process each element as a separate doc
Filter the separated docs from elements not containing flag
Group again by _id, and count the elements (that contain the flag field)
Filter by the new count_flags field
I don't really like this solution, it seems like a lot of steps to get something that should have basic support in a standard mongo find query.