How to match string within embedded array or doc in MongoDB? - mongodb

Question

After searching for a whole day, I am doubting whether MongoDB can fulfill below requirement:
Q: How can I filter out documents that meet below conditions ?
In last array element of students_replies, there is a reply from a student whose name containing string 'ason'.
id_1: first_school, students_replies: [
{Date:20210101, replies: [
{name: jack, reply: 'I do not like this idea'},
{name: jason, reply: 'I would rather stay at home'},
{name: charles, reply: 'I have an plan to improve'},
]},
{Date:20210401, replies: [
...]},
{Date:20210801, replies: [
...]},
]
id_2: second_shool, students_replies: [..]
id_3: third_shool, students_replies: [...]
Mongoplayground

Answer 1

Use $slice and $regex
For your example this becomes:
db.collection.aggregate([
// project only the last reply
{
"$project": {
key: 1,
last_reply: {
"$slice": [
"$students_replies",
-1
]
}
}
},
// filter the documents
{
"$match": {
"last_reply.replies.name": {
"$regex": "ason"
}
}
}
])
https://mongoplayground.net/p/a9piw2WQ8n6

Answer 2

Since you need last array element of students_replies, use $arrayElemAt
db.collection.aggregate([
{
"$match": {
$expr: {
$regexMatch: {
input: {
$reduce: {
input: {
$arrayElemAt: [
"$students_replies.replies",
-1
]
},
initialValue: "",
in: {
$concat: [
"$$value",
"$$this.name",
","
]
}
}
},
regex: "ason"
}
}
}
},
{
"$project": {
"students_replies": 0
}
}
])
mongoplayground
another answer
db.collection.aggregate([
{
$match: {
$expr: {
$ne: [
{
$filter: {
input: {
$map: {
input: {
$arrayElemAt: [
"$students_replies.replies",
-1
]
},
as: "r",
in: "$$r.name"
}
},
as: "s",
cond: {
$regexMatch: {
input: "$$s",
regex: "ason"
}
}
}
},
[]
]
}
}
},
{
"$project": {
"students_replies": 0
}
}
])
mongoplayground