Mongodb+mongoose update documents with date from other field = cast to date error

okay so i have some documents that are missing some data, im trying to update them like this;

await MyModel.updateMany(
  { imported: null },
  { $set: { 
    "imported": "$post.date"
  }}
)

but i get cast to date error.

so i decided to work with a single document to figure out why that is.

so first lets see what inside post.date

console.log(
  await MyModel.find({ _id: '5c2ce0fa527ad758bdb29506' })
               .select('post.date imported')
)

[
  {
    post: { date: 2018-11-29T18:02:25.000Z },
    _id: new ObjectId("5c2ce0fa527ad758bdb29506")
  }
]

okay, so i though what happens if i copy the date and hardcode it, like so:

await MyModel.updateMany(
  { _id: "5c2ce0fa527ad758bdb29506" },
  { $set: { 
    "imported": "2018-11-29T18:02:25.000Z"
  }}
)

that works just fine, but when i try to use the value from $post.date, like this:

await MyModel.updateMany(
  { _id: "5c2ce0fa527ad758bdb29506" },
  { $set: { 
    "imported": "$post.date"
  }}
)

it results in

Cast to date failed for value "$post.date" (type string) at path "imported"

i have tried to use $dateFromString and $toDate but with no luck, but i feel like mongoose is trying to validate the string value "$post.date" and not interpret it as mongo would even through the mongoose schema has no validators defined.

so i tried adding { runValidators: false }, but it feels like it is ignored

await MyModel.updateMany(
  { _id: "5c2ce0fa527ad758bdb29506" },
  { $set: { 
    "imported": "$post.date"
  }},
  { runValidators: false }
)

as the error is the same.

im running mongoose v. 6.2.0 and mongodb v. 4.2.18

any inputs are much appreciated

>Solution :

You can use this

db.collection.update({
  imported: null
},
[ //aggregate update
  {
    "$set": {
      "imported": "$post.date"
    }
  }
])

Supported from Mongo 4.2+