mongoid query rails 3 - ruby-on-rails-3

Question

brand new to MongoDB / mongoid and still pretty green on ruby / rails
on the rails console:
t = Task.all(:conditions => {:title => "task2"})
=> #<Mongoid::Criteria
selector: {:title=>"task2"},
options: {},
class: Task,
embedded: false>
>> t.first
=> #<Task _id: 4d7e2cdb73ec3227dd000009, title: "task2", latitude: 23.987904829, longitude: -12.9423875, created_by: "4d792d6973ec3248e3000001">
returns what I would expect, but
t = Task.where(:created_by => "4d792d6973ec3248e3000001")
=> #<Mongoid::Criteria
selector: {:created_by=>BSON::ObjectId('4d792d6973ec3248e3000001')},
options: {},
class: Task,
embedded: false>
>> t.first
=> nil
same result with:
t = Task.all(:conditions => {:created_by => "4d792d6973ec3248e3000001"})
here is the model:
class Task
include Mongoid::Document
field :title
field :latitude, :type => Float
field :longitude, :type => Float
field :created_by
end
what am I doing wrong ?
updated example:
>> Task.new(:title => "task8", :created_by => "4d792d6973ec3248e3000001")
=> #<Task _id: 4d81037973ec32cc22000003, latitude: nil, longitude: nil, title: "task8", created_by: "4d792d6973ec3248e3000001">
>> t = Task.all(:conditions => {:created_by => "4d792d6973ec3248e3000001"})
=> #<Mongoid::Criteria
selector: {:created_by=>BSON::ObjectId('4d792d6973ec3248e3000001')},
options: {},
class: Task,
embedded: false>
>> t.first
=> nil
do I need to specifically define created_by as :type => String ?

Answer 1

Looks like created_by is stored as a string in your document, but then you query looking for an ObjectId. ObjectId's and strings aren't the same.
Try changing
>> Task.new(:title => "task8", :created_by => "4d792d6973ec3248e3000001")
to
>> Task.new(:title => "task8", :created_by => BSON::ObjectId("4d792d6973ec3248e3000001"))

Answer 2

You have to include this functionality in mongoid as explained in docs:
class Person
include Mongoid::Document
include Mongoid::Timestamps
end