Group documents using substring of a field - regex

Question

I am working with MongoDB and I am enjoying a lot!
There is one query I am having problems to work with:
I have this set of data that represents an hierarchy (a tree, where 1 is the root, 1.1 and 1.2 are children of 1, and so on)
db.test.insert({id:1, hierarchy:"1"})
db.test.insert({id:2, hierarchy:"1.1"})
db.test.insert({id:3, hierarchy:"1.2"})
db.test.insert({id:4, hierarchy:"1.1.1"})
db.test.insert({id:5, hierarchy:"1.1.2"})
db.test.insert({id:6, hierarchy:"1.2.1"})
db.test.insert({id:7, hierarchy:"1.2.2"})
db.test.insert({id:8, hierarchy:"1.2.3"})
So if I make a query:
> db.test.find()
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4760"), "id" : 1, "hierarchy" : "1" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4761"), "id" : 2, "hierarchy" : "1.1" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4762"), "id" : 3, "hierarchy" : "1.2" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4763"), "id" : 4, "hierarchy" : "1.1.1" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4764"), "id" : 5, "hierarchy" : "1.1.2" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4765"), "id" : 6, "hierarchy" : "1.2.1" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4766"), "id" : 7, "hierarchy" : "1.2.2" }
{ "_id" : ObjectId("546a6095cafd2fa3ff8e4767"), "id" : 8, "hierarchy" : "1.2.3" }
the document with id 1 represents the CEO and I would like to gather information about the teams under the VPs (1.1 and 1.2).
I would like to have an output like this
{
id: null,
teams:
[
{
manager: 2,
hierarchy: "1.1",
subordinates: 2
},
{
manager: 3,
hierarchy: "1.2",
subordinates: 3
}
]
}
I am having problems to aggregate the documents in the right "slot".
I tried to use a regex to aggregate using the substring, and project before grouping and create a new field which would be "manager_hierarchy", so I could group using this field. But with none of these approaches I had any kind of success, so I am stuck now.
Is there anyway I could accomplish this task?
EDIT: sorry, I forgot to make one thing explicit:
This query is to get information about subordinate teams of an employee. I've used an example as the CEO, but if I was the employee 1.2.3 in the hierarchy, I would like to see the teams 1.2.3.1, 1.2.3.2, ..., 1.2.3.xx
There is also the possibility (rare, but possible) that someone has more than 9 subordinates, so making a "hardcoded" substring would not work, since
$substr:["$hierarchy",0,3]}
would work for 1.2 but not for 1.10
and
$substr:["$hierarchy",0,4]}
would work for 1.10, but not for 1.2

Answer 1

You can get your results using the below aggregate pipeline operations.
Sort the rows based on their hierarchy, so that the manager
comes on top.
Group together records that start with similar ancestors.(i.e 1.1
or 1.2,...). The manager record will be on top for each group
due to the above sort operation.
Take the count of each group, so the number of subordinates
will be count-1(the manager record).
Again group the records to get a single array.
The code:
db.test.aggregate([
{$match:{"id":{$gt:1}}},
{$sort:{"hierarchy":1}},
{$group:{"_id":{"grp":{$substr:["$hierarchy",0,3]}},
"manHeir":{$first:"$hierarchy"},
"count":{$sum:1},"manager":{$first:"$id"}}},
{$project:{"manager":1,
"hierarchy":"$manHeir",
"subordinates":{$subtract:["$count",1]},"_id":0}},
{$group:{"_id":null,"teams":{$push:"$$ROOT"}}},
{$project:{"_id":0,"teams":1}}
])