Rails form with nested attributes in has_many through relationship

Question

I'm attempting to build a form with multiple nested objects in a has_many :through relationship. However, the nested objects do not appear to be getting assigned the foreign_key of the parent object upon save. I'm unsure what I'm doing wrong here.

# app/models/admin/host.rb
class Admin::Host < ApplicationRecord
  has_many :admin_host_users, class_name: 'Admin::HostUser', foreign_key: 'admin_host_id', dependent: :destroy
  has_many :users, through: :admin_host_users

  accepts_nested_attributes_for(:admin_host_users, allow_destroy: true)

# app/models/admin/host_user.rb
class Admin::HostUser < ApplicationRecord
  belongs_to :admin_host, class_name: 'Admin::Host'
  belongs_to :user

# app/models/user.rb
class User < ApplicationRecord
  has_many :admin_host_users, class_name: 'Admin::HostUser'
  has_many :admin_hosts, through: :admin_host_users

# app/controllers/hosts_controller.rb
class HostsController < ApplicationController
  def create
    @host = Admin::Host.new(admin_host_params.merge(status: :pending))
    @host.save!
  end

  def new
    @host = Admin::Host.new
    @admin_host_users = [@host.admin_host_users.build(user: current_user)]
  end

  private
  
  def admin_host_params
    params.require(:admin_host).permit(
      :name,
      admin_host_users_attributes: [
        :user_id,
        :role,
      ],
    )
  end

# app/views/hosts/_form.html.erb
<%= form_with(model: @host, url: hosts_path, local: true) do |f| %>
  <% @admin_host_users.each do |admin_host_user| %>
    <%= f.fields_for(:admin_host_users, admin_host_user) do |admin_host_user_form| %>
      <%= admin_host_user_form.select(:user_id, @all_users) %>

Ok, that's the code. Here's the response when submitting the form:

Started POST "/hosts" for ::1 at 2021-05-23 08:45:22 -0400
Processing by HostsController#create as HTML
  Parameters: {"utf8"=>"✓", "admin_host"=>{"name"=>"Test", "admin_host_users_attributes"=>{"0"=>{"user_id"=>"1"}}}, "commit"=>"Apply Now"}

Completed 422 Unprocessable Entity in 287ms (ActiveRecord: 42.5ms)

ActiveRecord::RecordInvalid (Validation failed: Admin host users admin host must exist):
  
app/controllers/hosts_controller.rb:10:in `create'

Answer 1

Try setting the inverse_of option on the has_many associations

  has_many :admin_host_users, class_name: 'Admin::HostUser', foreign_key: 'admin_host_id', dependent: :destroy, inverse_of: 'Admin::Host'
  has_many :users, through: :admin_host_users, inverse_of: 'Admin::Host'