get the number of children in each nodein mongodb

Question

Recently I write an application that return relation between users.

I have a collection where documents are linked by parent_id :

{
 _id:1,
 parent_id:0
}
{
 _id:2,
 parent_id:1
}
{
 _id:3,
 parent_id:1
}
{
 _id:4,
 parent_id:2
}
{
_id:5,
 parent_id:2
}
{
_id:6,
 parent_id:4
}
{
_id:7,
 parent_id:4
}
and more...

So how can I return data Like this: I use laravel and mongodb

{
 _id:1
children:{
        {_id:2,
         parent_id:1
         children:{
                 {
                  _id:4,
                  parent_id:2
                  children:{
                      {
                       _id:6,
                       parent_id:4
                      }
                      {
                       _id:7,
                       parent_id:4
                      }
                  }
                  count:2 //Total number of children to End of This Node
                 }
                 {
                  id:5,
                  parent_id:2
                 }
                }
          count:4 //Total number of children to End of This Node
        }
        {
         _id:3,
         parent_id:1
        }
},
count:6 //Total number of children to End of This Node
}

---

How can I get the number of children in each node by aggregate

Thanks everyone:)