Use $Redact with a regular expression - mongodb

Question

In an aggregation pipeline, I am trying to filter some elements of an array of objects, based on the value of a field in this object.
Let's say that I have this entry:
{
"_id": "5b8911d346d19645f8a66bf4",
"title": "test task",
"creation_date": "2018-08-31T10:00:51.598Z",
"logs": [
{
"_id": "5b89126c46d19645f8a66bfb",
"content": "Running"
},
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopping"
},
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopped"
}
]
}
My objectif is to filter only the logs containing the stop word in their content:
{
"_id": "5b8911d346d19645f8a66bf4",
"title": "test task",
"creation_date": "2018-08-31T10:00:51.598Z",
"logs": [
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopping"
},
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopped"
}
]
}
I tried to use $redact to eliminate all the logs that does not contain the stop word:
$redact: {
$cond: {
if: { $match: { "logs.content": { $regex: "stop", $options: 'i' }}},
then: "$$KEEP",
else: "$$PRUNE"
}
}
but I keep getting the error:
Unrecognized expression '$match'

Answer 1

You can try below aggregation
db.collection.aggregate([
{ "$addFields": {
"logs": {
"$filter": {
"input": "$logs",
"cond": {
"$ne": [
{ "$indexOfBytes": [
{ "$toUpper": "$$this.content" },
{ "$toUpper": "stop" }
]},
-1
]
}
}
}
}}
])
Output
[
{
"_id": "5b8911d346d19645f8a66bf4",
"creation_date": "2018-08-31T10:00:51.598Z",
"logs": [
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopping"
},
{
"_id": "5b89128646d19645f8a66bfd",
"content": "Stopped"
}
],
"title": "test task"
}
]

Answer 2

As per your requirement below query is working and it is properly tested
db.users.aggregate(
// Pipeline
[
// Stage 1
{
$unwind: {
path : "$logs",
preserveNullAndEmptyArrays : true // optional
}
},
// Stage 2
{
$group: {
_id: "$_id",
"title" :{$last:"$title"} ,
"creation_date" :{$last:"$creation_date"},
logs: {
$push: {
$cond: [ {$or:[{"$eq":[{ "$substr": [ "$logs.content", 0, 4 ] }, "Stop"]},{"$eq":[{ "$substr": [ "$logs.content", 0, 4 ] }, "stop"]}]},{"_id":"$logs._id","content":"$logs.content"},null]
}
}
}
},
// Stage 3
{
$project: {
logs: {
$filter: {
input: "$logs",
as: "log",
cond: { $ne: [ "$$log", null ] }
}
}
}
},
]
// Created with Studio 3T, the IDE for MongoDB - https://studio3t.com/
);