Why elem_match is returning 0 elements?

Question

I am trying to get one record result from a collection of objects, but after following the Mongoid documentation I don't know what more to try.

I have this single element:

 > contacts
 => #<Contact _id: 55ace6bc6xx, device_fields: {"app_id"=>"55ace6bc65195efc8200xxxx"}, created_at: 2015-07-20 12:17:00 
    UTC, updated_at: 2015-07-20 12:17:00 UTC, name_first: "Kory", 
    name_last: "Funk", ...>

this list of matchers:

> apps = []
> apps << App.where(id: "55ace6bc65195efc8200xxxx").first.id
=> ["55ace6bc65195efc8200xxxx"] 

And this code trying to get the elements that match:

> contacts.elem_match(device_fields: {:app_id.in => apps }).to_a
=> []
> contacts.elem_match(device_fields: { "app_id": "55ace6bc65195efc8200xxxx"}).to_a
=> []

Why is it returning an empty array it there is one that matches?

Answer 1

According to official mongodb manual

The $elemMatch operator matches documents that contain an array field

And you are trying to use it with the hash field so you basically misunderstood this selection. So there is no object that matches.

Instead of it you should do:

contacts.where(:'device_fields.app_id'.in => apps).to_a