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I'm looking at MongoDB's documentation on the $ and $elemMatch
projections. I'm trying to figure out how to return only a subset of a
projection array's fields, but I cannot seem to figure it out.
Related posts:
I am not trying to perform a $slice from mongodb aggregation framework - Fetch first document's field of the nested array.
Nor am I trying to flatten the sub-document from Return only array value in mongo projection because I still want some fields from the top document.
Say I have the following documents in the test collection:
{
"_id": "A",
"array": [
{"key": 1, "name": "foo", "data": {}},
{"key": 2, "name": "bar", "data": {}}
],
"extra": {}
},
{
"_id": "B",
"array": [
{"key": 3, "name": "spam", "data": {}},
{"key": 4, "name": "eggs", "data": {}}
],
"extra": {}
}
The query I effectively want to perform is:
db.test.findOne({"array.key": 1}, {"array.$.name": 1, "extra": 1})
Which I would expect it to only return name under the sub-document in the
array where key was 1. E.g.,
{
"_id": "A",
"array": [
{"name": "foo"}
],
"extra": {}
}
But if I perform that query, I get this instead:
{
"_id": "A",
"array": [
{"key": 1, "name": "foo", "data": {}}
],
"extra": {}
}
Which is identical to doing the query:
db.test.findOne({"array.key": 1}, {"array.$": 1, "extra": 1})
I've also tried the following which results in the same:
db.test.findOne({"array.key": 1}, {"array.$": 1, "array.name": 1, "extra": 1})
Is there a way to only return a subset of the fields for array.$
instead of the whole sub-document?
348
You can select a single field in MongoDB using the following syntax: db. yourCollectionName. find({"yourFieldName":yourValue},{"yourSingleFieldName":1,_id:0});
To search the array of object in MongoDB, you can use $elemMatch operator. This operator allows us to search for more than one component from an array object.
When you use methods such as find() or findOne() in MongoDB, by default you get the whole document returned. And if you use projections, you can return specific key/value pairs. But what if you only want the value? You can extract the value of a field by appending that field's name to your query when using findOne() .
To exclude the _id field from the output documents of the $project stage, specify the exclusion of the _id field by setting it to 0 in the projection document.
Are you specifically trying to do this without using aggregate?
db.test.aggregate([{$unwind:"$array"},
{$match:{"array.key":1}},
{$project:{"array.name":1, extra:1}}])
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