Wrap Urn method next in fiber block

To protect against FiberError

app/models/urn.rb

@@ -27,10 +27,11 @@ def initialize
         teacher: "TE",
         salaried_trainee: "ST",
       }.with_indifferent_access
-      @default_urns = ->(_) { [] }
+      @default_urns = { teacher: [], salaried_trainee: [] }.with_indifferent_access
+      @default_existing_urns = -> { [] }
     end
 
-    attr_writer :prefix, :codes, :max_suffix, :seeds, :urns, :padding_size
+    attr_writer :prefix, :codes, :max_suffix, :seeds, :existing_urns, :padding_size
 
     def prefix
       @prefix || @default_prefix
@@ -53,7 +54,11 @@ def seeds
     end
 
     def urns
-      @urns || @default_urns
+      (@urns || @default_urns).with_indifferent_access
+    end
+
+    def existing_urns
+      @existing_urns || @default_existing_urns
     end
   end
 
@@ -69,19 +74,29 @@ def config
     end
 
     def next(route)
-      routes[route].next
+      Fiber.new { routes[route].next }.resume
     rescue KeyError
       raise(ArgumentError, "Invalid route: #{route}, must be one of #{config.codes.keys}")
     end
 
+    def load_existing_urns
+      return @urns if @urns
+
+      data = config.existing_urns.call
+      @urns = data.each_with_object(config.urns) do |(route, urn), hash|
+        hash[route] << urn
+        hash
+      end
+    end
+
   private
 
     def routes
       @routes ||= Concurrent::Hash.new do |hash, route|
         hash[route] = urn_enumerator(
           config.codes.fetch(route),
           config.seeds.fetch(route, Random.new_seed),
-          config.urns.call(route),
+          config.urns.fetch(route),
         )
       end
     end

app/services/submit_form.rb

@@ -27,6 +27,7 @@ def success?
   end
 
   def submit_form!
+    Urn.load_existing_urns
     create_application_records
     send_applicant_email
     @success = true

config/initializers/urn.rb

@@ -3,5 +3,5 @@
 Urn.configure do |c|
   c.prefix = "IRP"
   c.max_suffix = 99_999
-  c.urns = ->(route) { Application.where(application_route: route).pluck(:urn) }
+  c.existing_urns = -> { Application.group(:application_route, :urn).pluck(:application_route, :urn) }
 end