Use the value of a key as a key in mongodb aggregation - database

Question

I am working on a project where user can upload a *.csv* file to enter user data, also he can choose which one is the email field (user may have used Email, email, E-mail or something else as the email field).
Previous Developer saved the data in following format.
[
{
"_id" : ObjectId("5de7d0d65223850135eac968"),
groupName:"AGroup",
primaryField:"EMail",
FieldsData:[
{EMail:"abc#gmail.com",otherField:123},
{EMail:"def#outlook.com",otherField:345}
]
},
{
"_id" : ObjectId("5de7d0d65223850135eac969"),
groupName:"BGroup",
primaryField:"userEmail",
FieldsData:[
{userEmail:"hij#hotmail.com",otherField:678},
{userEmail:"kl#outlook.com",otherField:910}
]
}
]
here FieldData is the data from .csv file and primaryField is the field user have chosen as the email field .
I have to get the primaryField name and get the corresponding field value from FieldData.
For Example: For first group i.e.. AGroup primaryField is EMail hence the mailList will contain the value of FieldData.EMail
Right now I have to do this in following steps:
Get the primaryField of all the groups
db.Group.aggregate({$project:{_id:1,PrimaryKey:"$primaryField"}})
Loop over all the data
Make query with each groupId and primaryField
db.Group.aggregate({$match:{ "_id" : ObjectId("5de7d0d65223850135eac968")}},
{$project:{MailList:"$FieldsData.EMail"}})
Please suggest a way to do the job in one query. Basically I need the data in following format
[
{ "_id" : ObjectId("5de7d0d65223850135eac968"), "MailList" : [ "abc#gmail.com", "def#outlook.com" ] }
{ "_id" : ObjectId("5de7d0d65223850135eac969"), "MailList" : [ "hij#hotmail.com", "kl#outlook.com" ] }
]

Answer 1

To read and compare object keys dynamically you need to use $objectToArray. You can use $let to define temporary variable element which will hold first found k-vpair matching your PrimaryField and then return v part.
db.collection.aggregate([
{
$project: {
_id: 1,
MailList: {
$map: {
input: "$FieldsData",
in: {
$let: {
vars: {
element: {
$arrayElemAt: [
{
$filter: {
input: { $objectToArray: "$$this" },
cond: {
$eq: [ "$$this.k", "$primaryField" ]
}
}
}, 0
]
}
},
in: "$$element.v"
}
}
}
}
}
}
])
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