I have a document like this:
{
value: "field2",
field1: [ ... ],
field2: [ ... ],
...
}
Where value will be the value of one of the fields in the document. and many different fields are possible for one document.
I want to match a document. fetch the relevant field only and them do some calculations on it.
For example I want to do:
{
$unwind: "$value"
}
And get the results of field2 unwinded.
How can I do this?
It's a little bit "hacky" but you can achieve this using operators like $objectToArray and $filter like so:
db.collection.aggregate([
{
$addFields: {
"values": {
$arrayElemAt: [
{
$filter: {
input: {
$objectToArray: "$$ROOT"
},
as: "field",
cond: {
$eq: [
"$$field.k",
"$value"
]
}
}
},
0
]
}
}
},
{
$unwind: "$values.v"
},
{
$replaceRoot: {
newRoot: "$values.v"
}
},
])