How to update a subfield element from another subfield element in mongodb? - mongodb

Question

Image this data structure :
{
_id: 1
...
sections: [
{
Title: 'hello world',
Title2: '',
},
{
Title: 'goodbye',
Title2: ''
}
]
}
I need to update all Title2 from Title.
I tried things like :
db...update(
{ },
[{ $set:
{
"sections.Title2": "sections.Title",
}
}])
but without success. Also tried with updateMany and some variants like sections.$.Title2.
Thank you for any help

Answer 1

You can do via update + aggregation pipleine(mongod 4.2+) & $map as follow:
db.collection.update({
sections: {
$exists: true
}
},
[
{
$addFields: {
"sections": {
$map: {
input: "$sections",
as: "s",
in: {
"Title": "$$s.Title",
"Title2": "$$s.Title"
}
}
}
}
}
],
{
multi: true
})
Explained:
Find and replace the existing sections with mapped the necessary array values for Title2 to Title.
Add the option {multi:true} to update all documents in collection
playground
Improved version2:
db.collection.update({
sections: {
$exists: true
}
},
[
{
$addFields: {
"sections": {
$map: {
input: "$sections",
as: "s",
in: {
$mergeObjects: [
"$$s",
{
"Title2": "$$s.Title"
}
]
}
}
}
}
}
],
{
multi: true
})
Explained:
In this version you merge the changed value with the subdocument so you dont need to add every of the other fields that are nto going to be changed.
playground2