Add unrelated collections in rabl - ruby

Question

I have an employee controller which includes designation and department. Each one is separate model so i can access each api in the client side to fill each drop down. I need like view model concept in ASP.NET MVC.
For this i read work around like return 2 collections in one json object. But i am confused in the rabl file.
My controller code is below;
def employee_masters
#designations = Designation.all
#departments = Department.all
respond_with #designations
end
and my existing employee_masters.rabl is
collection :#designations
attributes :id, :name
please guide me how to change my rabl and controller code to achieve the behaviour. My expected json is below
{
"designations": [
{
"id": 1,
"name": "Program Manager"
},
{
"id": 2,
"name": "Project Manager"
},
{
"id": 3,
"name": "Tech Lead"
}
],
"departments": [
{
"id": 1,
"name": "IT"
},
{
"id": 2,
"name": "Support"
},
{
"id": 3,
"name": "Finance"
}
]
}
Modify:
I read this and got some idea. I created a class like below
class EmployeeViewModel
attr_accessor :departments, :designations
def initialize(departments, designations)
#departments = departments
#designations = designations
end
end
My controller code is
def employee_masters
#designations = Designation.all
#departments = Department.all
#employee_view_model = EmployeeViewModel.new(#departments, #designations)
respond_with #employee_view_model
end
And my rabl is
object #employee_view_model
child :departments do
extends 'api/v1/departments/show'
end
child :designations do
extends 'api/v1/designations/show'
end
Here shows error as Template::Error (undefined method `departments' for #<#:0x00000003872168>):
But when i change respond_with #employee_view_model to simply render :json => #employee_view_model it worked well and return the json. Please help to rectify rabl error