I have the following model:

class Activity
  belongs_to :source, polymorphic: true
end

The source can be e.g. a Fund.

I query activities:
activities = Activity.all
Then funds:
funds = Fund.where(:id.in => activities.pluck(:source_id).compact, name: /something/i)

The results are non-empty. Then I query:
Activity.in(source_id: funds.pluck(:id))
and the result is empty. When queried directly through the mongo console, the expected record is found.

Some actual results:
code
[22] pry(main)> activities.count
=> 10561
[23] pry(main)> funds.count
=> 1
[24] pry(main)> Activity.in(source_id: [f.first.id]).count
=> 0
[25] pry(main)> Activity.in(source_id: [f.first.id])
=> #<Mongoid::Criteria
selector: {"deleted_at"=>nil, "source_id"=>{"$in"=>[BSON::ObjectId('543d2abf62653157556fd200')]}}
options: {:sort=>{"date"=>-1}}
class: Activity
embedded: false>
[26] pry(main)> a.pluck(:source_id).include?(f.first.id)
=> false
[27] pry(main)> a.pluck(:source_id).map(&:to_s).include?(f.first.id.to_s)
=> true
[29] pry(main)> a.pluck(:source_id).include?(f.first.id.to_s)
=> true
code

Seems to be something about the ObjectId conversion?