Finding an element in array of ObjectId in MongoDB

Question

I have a list of ObjectId's mapped using Morphia. In java, the mapping looks like this:

public class Log {
    @Indexed
    public List<ObjectId> companyIds;
    ....
}

In Mongo shell, however, when I search using $elemMatch it will complain about invalid type.

> db.Log.find({ "companyIds" : { "$elemMatch" : ObjectId("5059e90d0364d02be740417a")}})
error: {
    "$err" : "invalid parameter: expected an object ($elemMatch)",
    "code" : 10065
}

Is there anything else I need to do to use $elemMatch with ObjectId's?

Thanks

jjm · Accepted Answer · 2012-09-20 02:01:59Z

10

You don't need to use $elemMatch to do this. You can just run the query

{ "companyIds" : ObjectId("...") }

which will find all objects whose companyIds field contains the given objectId.

According to the docs, $elemMatch is only necessary when you're trying to match against multiple fields on an array element. The reason it's saying "expected an object" is that $elemMatch takes a full fledged mongo query (as in, something that you could pass to find) as it's argument.

answered Sep 20, 2012 at 2:01

jjm

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