MongoDB shell version: 2.0.4
> show collections
find_by_models
system.indexes

> db.find_by_models.find({ f1: 1 })
{ "_id": ObjectId("511bb556bd35bb48d3000001"), "f1": 1, "f2": 2, "f3": 3 }
{ "_id": ObjectId("511bb556bd35bb48d3000002"), "f1": 1, "f2": 2, "f3": 3 }
{ "_id": ObjectId("511bb556bd35bb48d3000003"), "f1": 1, "f2": 2, "f3": 3 }
{ "_id": ObjectId("511bb556bd35bb48d3000004"), "f1": 1 }

[1] pry > clean_model.create!(f1: 1)
# => #<
  Lwhoxemrvrr
  _id: 511bb556bd35bb48d3000004,
  f1: 1,
  f2: nil,
  f3: nil,
  _type: nil
>

[2] pry > clean_model.find_by(f1: 1)
# => nil

[3] pry > clean_model.where(f1: 1)
# => #<
  Mongoid::Criteria
  selector: {"f1"=>1}
  options:  {}
  class:    Lwhoxemrvrr
  embedded: false
>

[4] pry > clean_model.where(f1: 1).to_a
# => []