ruby-on-rails - Simple where query returns no results with mongomapper

Question

I have a database with some data in a rails project with mongomapper.

In the rails console:

[1] pry(main)> MyObject.first
=> #<MyObject _id: BSON::ObjectId('525e6a9156c02c22de000002'), my_field: 1, 
...

[2] pry(main)> MyObject.where(my_field: 1).all
=> []
[3] pry(main)> MyObject.where(my_field: 1.to_s).all
=> []
[4] pry(main)> MyObject.where(my_field: "1").all
=> []

What am I missing ?