-Subjects
    -math
        id: 1
        name: Math
        -students
            -arya
                id: 1
                name: Arya
            -jonsnow
                id: 2
                name: JonSnow
            +justsomename
    +science
    +english
    +history
    +computer

ref.queryOrdered(byChild: "students/name").queryEqual(toValue: "Arya").observeSingleEvent(of: .value, with: { snapshot in

        print(snapshot)
    })

Snap (subjects) <null>