class User
  include Mongoid::Document
  has_many :comments
end

class Comment
  include Mongoid::Document
  belongs_to :user, optional: true
  field :time, type: Integer
end

user = User.create!
user.comments.and(Comment.or({:time.gt => 5}, {:time => nil})).selector

=> {"user_id"=>BSON::ObjectId('5e7498a4e1b8323c9851dba6'), "$and"=>[{"$or"=>[{"time"=>{"$gt"=>5}}, {"time"=>nil}]}]}

irb(main):014:0> Comment.where(foo: 1).and(Comment.or({:time.gt => 5}, {:time => nil})).selector 
=> {"foo"=>1, "$and"=>[{"$or"=>[{"time"=>{"$gt"=>5}}, {"time"=>nil}]}]}
irb(main):015:0> Comment.and(Comment.or({:time.gt => 5}, {:time => nil})).selector=> {"$and"=>[{"$or"=>[{"time"=>{"$gt"=>5}}, {"time"=>nil}]}]}