How can I sort based on children? - database

Question

Using Vapor and Fluent (PostgreSQL if that matters) I have entity B that has aID: Node (A is B's parent) to reference A and A has a one-to-many relationship with B. How can I make a query to fetch all A's sorted by the count of B's?
I want the result to look something like this:
All A's in DB
[
{
"id": 4,
"name": "Hi",
"bCount": 1000
},
{
"id": 3,
"name": "Another",
"bCount": 800
},
{
"id": 5,
"name": "Test",
"bCount": 30
}
]

Answer 1

Firstly,
Create a modal for A
Turn that JSON string to array of A
if you do this your sorting becomes as easy as -
array.sort { $0.bCount < $1.bCount }

Answer 2

This is going to be tricky to implement entirely in Fluent using Entity. Firstly, you will need to use raw SQL to get your bCount. Secondly, you will need to change your init(node:) to accept bCount, though it shouldn't be in your makeNode() because we don't want to create a stored database field for it.
Try this for your raw SQL (untested):
SELECT
A.*,
(
SELECT COUNT(*)
FROM B
WHERE B.aID = A.id
) AS bCount
FROM A
ORDER BY bCount
Then, run that query to get your A models.
var models: [A] = []
if let driver = drop.database?.driver as? PostgreSQLDriver {
if case .array(let array) = try driver.raw(sql) {
for result in array {
do {
var model = try A(node: result)
models.append(model)
}
}
}
}
As I said before, your init method on A will be receiving bCount so you will need to store it there.