MongoDB How to add a field to ALL embedded documents in an array

Question

If I have some documents inside a collection. In this format:

{posts: [{"name": "post1"},..., {"name": "postN"}]}

What is the equivalent of this function:

db.my_collection.update({}, { $set: {"posts.0.lastMod": new Date()}}, false, true)

But to update all the embedded documents instead of only position 0? I have tried doing:

db.my_collection.update({}, { $set: {"posts.lastMod": new Date()}}, false, true)

But that just gave me an error already on the first document:

WriteResult({
    "nMatched" : 0,
    "nUpserted" : 0,
    "nModified" : 0,
    "writeError" : {
        "code" : 28,
        "errmsg" : "Cannot create field 'lastMod' in element {posts: [ALL MY ARRAY WAS PRINTED HERE]"

Answer 1

The answer in the comments above is 99% there. This makes it a tad safer by ensuring it will only use the dotpath trick to add lastMod to an existing array of posts. If posts does not exist, without the predicate it will create a brand new single object posts which is probably not desired.

db.foo.updateMany(
    {posts: {$exists:true}},
    [
        { $addFields: { 'posts.lastMod': new ISODate() } }
    ]
);

Answer 2

Based on this documentation https://www.mongodb.com/docs/manual/reference/method/db.collection.updateMany/, you can add a aggregate pipeline to the 2nd param of updateMany, so you can try this query

db. my_collection.updateMany({},[ { $addFields: { 'posts.lastMod': new Date() } }, { $set: { } }]) because you dont want to add or update any properties else the posts.lastMod, so you can leave the $set as {}