Finding the collection details if one field is distinct - mongodb

Question

I have a database of teachers details as given
{ "_id" : ObjectId("5bcc0a44f2752576a8545d99"), "Teacher_id" : "Pic002", "Teacher_Name" : "Ravi Kumar", "Dept_Name" : "IT", "Salary" : 40000, "Status" : "A" }
{ "_id" : ObjectId("5bcc0a5af2752576a8545d9a"), "Teacher_id" : "Pic003", "Teacher_Name" : "Akshay", "Dept_Name" : "Comp", "Salary" : 25500, "Status" : "N" }
{ "_id" : ObjectId("5bcc0a85f2752576a8545d9b"), "Teacher_id" : "Pic003", "Teacher_Name" : "Akshay", "Dept_Name" : "Comp", "Salary" : 25500, "Status" : "N" }
{ "_id" : ObjectId("5bcc0a9af2752576a8545d9c"), "Teacher_id" : "Pic004", "Teacher_Name" : "Sumit", "Dept_Name" : "Mech", "Salary" : 35000, "Status" : "N" }
How would I list down complete details of a teacher whose Department Name is distinct?
Basically, I want to display the details of the first first and last document in this collection.

Answer 1

You can achieve this via this aggregation:
db.collection.aggregate([{
$group: {
_id: "$Dept_Name",
docs: {
$addToSet: "$$CURRENT"
},
count: {
$sum: 1
}
}
},
{
$match: {
"count": {
"$eq": 1
}
}
},
{
$unwind: "$docs"
},
{
$replaceRoot: {
newRoot: "$docs"
}
}
])
The idea is first to $group and at the same time keep the objects via $addToSet.
Then filter (via $match) on those which count is 1 and then $unwind & $replaceRoot.
See it working here