Mongoid, stranger mongodb query when uses belongs_to relation

Question

I'm using mongoid 6 and I have this two models in my project:

class User
  belongs_to :project
end

class Project
  has_many :users
end

When I access to project from user instance I see this mongodb log:

> user.project
... DEBUG -- : MONGODB ... STARTED | {"find"=>"projects", "filter"=>{"_id"=>BSON::ObjectId('5accdd5775aefb2d085087e2')}, "sort"=>{"_id"=>1}, "limit"=>1, "singleBatch"=>true}
... DEBUG -- : MONGODB ... SUCCEEDED | 0.000482198s

Why this uses sort and limit ? This doesn't happen when you find project manually:

> Project.find(user.project_id)
... DEBUG -- : MONGODB ... STARTED | {"find"=>"projects", "filter"=>{"_id"=>BSON::ObjectId('5accdd5775aefb2d085087e2')}}
... DEBUG -- : MONGODB ... SUCCEEDED | 0.000278153s

It seems slower and breaks the use of mongoid query caching is somes cases.