MongoDB Track data changes - database

Question

I want to track changes on MongoDB Documents. The big Challenge is that MongoDB has nested Documents.
Example
[
{
"_id": "60f7a86c0e979362a25245eb",
"email": "walltownsend#delphide.com",
"friends": [
{
"name": "Hancock Nelson"
},
{
"name": "Owen Dotson"
},
{
"name": "Cathy Jarvis"
}
]
}
]
after the update/change
[
{
"_id": "60f7a86c0e979362a25245eb",
"email": "walltownsend#delphide.com",
"friends": [
{
"name": "Daphne Kline" //<------
},
{
"name": "Owen Dotson"
},
{
"name": "Cathy Jarvis"
}
]
}
]
This is a very basic example of a highly expandable real world use chase.
On a SQL Based Database, I would suggest some sort of this solution.
The SQL way
users
_id
email
60f7a8b28db7c78b57bbc217
cathyjarvis#delphide.com
friends
_id
user_id
name
0
60f7a8b28db7c78b57bbc217
Hancock Nelson
1
60f7a8b28db7c78b57bbc217
Suarez Burt
2
60f7a8b28db7c78b57bbc217
Mejia Elliott
after the update/change
users
_id
email
60f7a8b28db7c78b57bbc217
cathyjarvis#delphide.com
friends
_id
user_id
name
0
60f7a8b28db7c78b57bbc217
Daphne Kline
1
60f7a8b28db7c78b57bbc217
Suarez Burt
2
60f7a8b28db7c78b57bbc217
Mejia Elliott
history
_id
friends_id
field
preUpdate
postUpdate
0
0
name
Hancock Nelson
Daphne Kline
If there is an update and the change has to be tracked before the next update, this would work for NoSQL as well. If there is a second Update, we have a second line in the SQL database and it't very clear. On NoSQL, you can make a list/array of the full document and compare changes during the indexes, but there is very much redundant information which hasn't changed.

Answer 1

Have a look at Set Expression Operators
$setDifference
$setEquals
$setIntersection
Be ware, these operators perform set operation on arrays, treating arrays as sets. If an array contains duplicate entries, they ignore the duplicate entries. They ignore the order of the elements.
In your example the update would result in
removed: [ {name: "Hancock Nelson" } ],
added: [ {name: "Daphne Kline" } ]
If the number of elements is always the same before and after the update, then you could use this one:
db.collection.insertOne({
friends: [
{ "name": "Hancock Nelson" },
{ "name": "Owen Dotson" },
{ "name": "Cathy Jarvis" }
],
updated_friends: [
{ "name": "Daphne Kline" },
{ "name": "Owen Dotson" },
{ "name": "Cathy Jarvis" }
]
})
db.collection.aggregate([
{
$set: {
difference: {
$map: {
input: { $range: [0, { $size: "$friends" }] },
as: "i",
in: {
$cond: {
if: {
$eq: [
{ $arrayElemAt: ["$friends", "$$i"] },
{ $arrayElemAt: ["$updated_friends", "$$i"] }
]
},
then: null,
else: {
old: { $arrayElemAt: ["$friends", "$$i"] },
new: { $arrayElemAt: ["$updated_friends", "$$i"] }
}
}
}
}
}
}
},
{
$set: {
difference: {
$filter: {
input: "$difference",
cond: { $ne: ["$$this", null] }
}
}
}
}
])