Something not right here with Kate.

app/controllers/credentials_controller.rb

@@ -1,5 +1,6 @@
 class CredentialsController < ApplicationController
   before_action :set_credential, only: %i[ show edit update destroy ]
+  before_action :set_domain, only: %i[ new ]
 
   # GET /credentials or /credentials.json
   def index
@@ -63,6 +64,10 @@ class CredentialsController < ApplicationController
       @credential = Credential.find(params[:id])
     end
 
+    def set_domain
+      @domain = Domain.find(params[:domain_id])
+    end
+
     # Only allow a list of trusted parameters through.
     def credential_params
       params.require(:credential).permit(:email, :password)

app/models/credential.rb

@@ -2,5 +2,6 @@ class Credential < ApplicationRecord
     belongs_to :domain
     validates :email, presence: true
     validates :email, uniqueness: true
+    validates :domain_id, presence: true
     validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i , message: "email must be valid"}
 end

app/models/virtual.rb

@@ -3,6 +3,7 @@ class Virtual < ApplicationRecord
     validate :domain_name_exists
     validates :email, presence: true
     validates :destination, presence: true
+    validates :domain_id, presence: true
     validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i , message: "email must be valid"}
     validates :destination, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i , message: "destination must be valid"}
     validates :destination, uniqueness: {scope: :email, message: "a redirection pair must be unique"}

app/views/credentials/_form.html.erb

@@ -1,4 +1,4 @@
-<%= form_with(model: credential) do |form| %>
+<%= form_with(model: [@domain, credential]) do |form| %>
   <% if credential.errors.any? %>
     <div style="color: red">
       <h2><%= pluralize(credential.errors.count, "error") %> prohibited this credential from being saved:</h2>

app/views/credentials/new.html.erb

@@ -5,5 +5,5 @@
 <br>
 
 <div>
-  <%= link_to "Back to credentials", credentials_path %>
+  <%= link_to "Back to the domain", domain_path(@domain) %>
 </div>

app/views/domains/show.html.erb

@@ -2,10 +2,7 @@
 <h1><%= @domain.domain %></h1>
 
 <div>
-  <%= link_to "Edit this domain", edit_domain_path(@domain) %> |
-  <%= link_to "Back to domains", domains_path %>
-
-  <%= button_to "Destroy this domain", @domain, method: :delete %>
+  <%= link_to "Edit this domain", edit_domain_path(@domain) %> | <%= link_to "Back to domains", domains_path %> | <%= button_to "Destroy this domain", @domain, method: :delete %>
 </div>
 
 <h2>Credentials (<%= @domain.credentials.count %>)</h2>