Mongoid batch insert not using key

Model.collection retrieves the internal (ruby driver) collection instead of using mongoid. Batch insert needs to be done that way because it is not supported by mongoid itself.

Essentially, by accessing the collection directly, you are bypassing mongoid and so the fact that you declare key :u_id doesn't (and shouldn't) have any effect.

The solution is to just use _id instead of u_id so that the driver doesn't try to generate one for you.

friends.each do |friend|
  friend[:_id] = friend["id"]
  friend.delete "id"
end

However, this may have implications elsewhere in your code.

Another solution, if you are lazy, is to just not care about the extra object_id and let the driver create one for you.