MongoDB (mongoose): Cast to [string] failed for value - mongodb

Question

There's something wrong happens when I try to update a document in DB.
Here is Schema:
const prodSchema = new Schema({
name: {
type: String,
required: true,
unique: true
},
description: {
type: [String]
},
})
Then I get some product from elsewhere:
const some_product = axios.get(blah blah)
And update mu document.
Note that I set up a condition for a 'description': if it has a null value, it is updated - else it remains the same:
const newProduct = {
$set: {
name: some_product.name,
description: {
$cond: {
if: {
$eq: [null, '$description']
},
then: [some_product.description],
else: '$description'
}
}
}
}
Go update (I use mongoose):
Product.updateOne({name: some_product.name}, newProduct, {some params})
And I see this:
Cast to [string] failed for value
"[{"$cond":{"if":{"$eq":[null,"$description"]},"then":["Here is some
new description of the prosuct"],"else":"$description"}}]" at path
"description"
I think the point is that the type of description in the schema is array of strings, and 'description' field in the requested 'some_product' is just a string. May be this is the issue.
But how do I solve it?
Many thanks.

Answer 1

var description = []
description.push(some_product.description)
const newProduct = [{
$set: {
name: some_product.name,
description: {
$cond: {
if: {
$eq: [null, '$description']
},
then: description,
else: '$description'
}
}
}
}]
put update operation in [] use update pipeline
for example
model.update({},[{$set: ...}])