MongoDB search using $in array not working - arrays

Question

I'm using MongoDB shell version: 2.4.8, and would simply like to know why a nested array search doesn't work quite as expected.
Assume we have 2 document collections, (a) Users:
{
"_id" : ObjectId("u1"),
"username" : "user1",
"org_ids" : [
ObjectId("o1"),
ObjectId("o2")
]
}
{
"_id" : ObjectId("u2"),
"username" : "user2",
"org_ids" : [
ObjectId("o1")
]
}
and (b) Organisations:
{
"_id" : ObjectId("o1"),
"name" : "Org 1"
}
{
"_id" : "ObjectId("o2"),
"name" : "Org 2"
}
Collections have indexes defined for
Users._id, Users.org_id, Organisations._id
I would like to find all Organisations a specific user is a member of.
I've tried this:
> myUser = db.Users.find( { _id: ObjectId("u1") })
> db.Organisations.find( { _id : { $in : [myUser.org_ids] }})
yet it yields nothing as a result. I've also tried this:
> myUser = db.Users.find( { _id: ObjectId("u1") })
> db.Organisations.find( { _id : { $in : myUser.org_ids }})
but it outputs the error:
error: { "$err" : "invalid query", "code" : 12580 }
(which basically says you need to pass $in an array) ... but that's what I thought I was doing originally ? baffled.
Any ideas what I'm doing wrong?

Answer 1

db.collection.find() returns a cursor - according to documentation. Then myUser.org_ids is undefined, but $in field must be an array. Let's see the solution!
_id is unique in a collection. So you can do findOne:
myUser = db.Users.findOne( { _id: ObjectId("u1") })
db.Organisations.find( { _id : { $in : myUser.org_ids }})
If you are searching for a non-unique field you can use toArray:
myUsers = db.Users.find( { username: /^user/ }).toArray()
Then myUsers will be an array of objects matching to the query.