get difference between two dates in $project - json

Question

Trying to project the date difference between two dates, but I am getting error -
Invalid $project :: caused by :: Unknown expression $dateDiff
db.books.aggregate([{
$project:{
Date_diff:
{$dateDiff:{
start_dt:'$borrowers_list.borrowed_dt',
endDate:'$borrowers_list.return_dt',
unit: "day"
}
}
}
}])
The json document structure is like this -
_id:6188a5283543f7cc2f77c73f
branch_id:1
borrowers_list:Object
0:Object
borrowed_dt:2021-08-15T06:00:00.000+00:00
card_no:"ID000067"
return_dt:2021-08-25T06:00:00.000+00:00
I have no idea why the error is unknown expression $dateDiff, as my syntax is correct. Does anyone have any suggestions?

Answer 1

Based on your provided JSON document, the document should be as below (correct me if it is incorrect):
{
_id: ObjectId("6188a5283543f7cc2f77c73f"),
branch_id: 1,
borrowers_list: {
0: {
borrowed_dt: ISODate("2021-08-15T06:00:00.000+00:00"),
card_no: "ID000067",
return_dt: ISODate("2021-08-25T06:00:00.000+00:00")
}
}
}
]
There is no start_dt in $dateFiff field, it is startDate.
Query
db.collection.aggregate([
{
$project: {
Date_diff: {
$dateDiff: {
startDate: "$borrowers_list.0.borrowed_dt",
endDate: "$borrowers_list.0.return_dt",
unit: "day"
}
}
}
}
])
Note: Above query will perform the $dateDiff for the first document in borrowers_list.
Sample Mongo Playground
In case you need to iterate each document (with key-value pair) in borrowers_list to perform $dateDiff.
$set - Convert from object to array (via $objectToArray) for borrowers_list to new field borrowers.
$set - Iterate each document in borrowers array (1) and perform $dateDiff.
$project - Decorate the output document, convert Date_diff from array to object (via $objectToArray).
Query
db.collection.aggregate([
{
$set: {
borrowers: {
"$objectToArray": "$borrowers_list"
}
}
},
{
$set: {
Date_diff: {
$map: {
input: "$borrowers",
as: "b",
in: {
k: "$$b.k",
v: {
$dateDiff: {
startDate: "$$b.v.borrowed_dt",
endDate: "$$b.v.return_dt",
unit: "day"
}
}
}
}
}
}
},
{
$project: {
Date_diff: {
"$arrayToObject": "$Date_diff"
}
}
}
])
Sample Mongo Playground (Iterate document with key-value pair)