mongodb findAndModify update element in array - mongodb

Question

There is an bson document:
{
"_id" : ObjectId("5718441f5116a60b08000b8c"),
"mails" : [
{
"id" : 2,
"a" : [
{
"a" : 1
},
{
"a" : 2
}
]
},
{
"id" : 1,
"a" : [
{
"a" : 1
},
{
"a" : 2
}
]
}
]
}
I need to return and clear the array "a" which belong to "mails.id == x" for given document. So I use findAndModify like:
db.mail.findAndModify({query: {"_id":ObjectId("5718441f5116a60b08000b8c")}, update: {$set:{"mails.$.a":[]}}, new: false, fields:{"mails":{$elemMatch:{"id":1}}}})
However this don't work. The problem is the $set should apply on one document in array rather than the whole document. So I need a projection to project it out.
If I left update to blank, it will return the desired part:
{
"_id" : ObjectId("5718441f5116a60b08000b8c"),
"mails" : [
{
"id" : 1,
"a" : [
{
"a" : 1
},
{
"a" : 2
}
]
}
]
}
But I don't know how to clear the array 'a' in 'mails'

Answer 1

You have to specify array element match in the query:
db.mail.findAndModify({query: {"_id":ObjectId("5718441f5116a60b08000b8c"), "mails":{$elemMatch:{"id":1}}}, update: {$set:{"mails.$.a":[]}}, new: false, fields:{"mails":{$elemMatch:{"id":1}}}})
mails.$ in you update matches the first matched element in the doc, so you have to match it in the query. Also, this query will update the doc, but it will return the old version, since you use new: false, if you want to get the updated version set it to true.