How to get a child of a mongo collections by the key?

Question

I ve got a collection users that look like this :

{
_id: myid,
name : 'blabla',
ia : {
       [0]-> 'an id',
       [1]-> 'a second id'
      }
}

And i want to have only my first id of ia, so i tried something like that :

User.find({ _id: id, ia :{ key : indexia} }, ['ia']).populate('ia').run(rendu);

where id= myid, and indexia=0.

If i don t put that part : " ia :{ key : indexia}" i get all my ia... but i want only one.

Hope i was clear. Thanks

Answer 1

The MongoDB find() command is intended to match an entire document.

When you run the following query

{ _id: id, ia :{ key : indexia} }

... this means "where _id equals id and ia equals the object {key:0}". That's probably not what you want.

If i don t put that part : " ia :{ key : indexia}" i get all my ia... but i want only one.

It sounds like you want to return a piece of a given document. The documentation for that is here. In your case it would look like this:

find({ _id: id}, { 'ia.0' : 1 })

The first part of the find will find the document. The second part will return specific fields.

Answer 2

Your collection appears to have the schema:

{ "_id" : "id",
  "ia"  : ["id1","id2","id3"],
   etc.
}

In this case, the query to get just the first "id" of the "ia" field would be:

find({}, { "ia" : {$slice:1  } })

You can read more about $slice operator here. Mongoose documents its support for the $slice operator on the query page. Hope this helps!

Answer 3

Thanks for your help It worker like that:

User.find({ _id: id }).only('ia').slice('ia', indexia, 1).populate('ia').run(rendu);

The only('ia') don t work, but not a big deal.