RSpec.describe Api::V1::AreaService do
    Service=Api::V1::AreaService // I will be using Service instead of Api::V1::AreaService
    it 'should save a new area' do
        expect {
            Service.add 'some', 'area' // This is one example
        }.to change(Area, :count).by(1)
    end
end

class MyModule::AreaController < MyModule::MainController
    Service = Api::V1::AreaService
    def add
        area = params['area']
        city = params['city']
        Service.add area, city
    end
end