Added test case for belongs_to association error displaying.

demo/app/controllers/addresses_controller.rb

@@ -0,0 +1,9 @@
+class AddressesController < ApplicationController
+  def create
+    redirect_to root_path
+  end
+
+  def update
+    redirect_to root_path
+  end
+end

demo/config/routes.rb

@@ -1,5 +1,6 @@
 Dummy::Application.routes.draw do
   get "fragment" => "bootstrap#fragment", as: :fragment
+  resources :addresses
   resources :users
 
   root to: "bootstrap#form"

test/bootstrap_fields_test.rb

@@ -100,6 +100,22 @@ class BootstrapFieldsTest < ActionView::TestCase
     assert_equivalent_html expected, bootstrap_form_for(@user) { |f| f.file_field(:misc) }
   end
 
+  test "errors are correctly displayed for belongs_to assoication fields" do
+    @address.valid?
+
+    expected = <<~HTML
+      <form accept-charset="UTF-8" action="/addresses" class="new_address"  id="new_address" method="post">
+        #{'<input name="utf8" type="hidden" value="&#x2713;"/>' unless ::Rails::VERSION::STRING >= '6'}
+        <div class="mb-3">
+          <label class="form-label required" for="address_user_id">User</label>
+          <input aria-required="true" class="form-control is-invalid" id="address_user_id" name="address[user_id]" required="required" type="text"/>
+          <div class="invalid-feedback">must exist</div>
+        </div>
+      </form>
+    HTML
+    assert_equivalent_html expected, bootstrap_form_for(@address) { |f| f.text_field(:user_id) }
+  end
+
   test "hidden fields are supported" do
     expected = <<~HTML
       <input #{autocomplete_attr} id="user_misc" name="user[misc]" type="hidden" />

test/test_helper.rb

@@ -12,6 +12,7 @@
 
 class ActionView::TestCase
   def setup_test_fixture
+    @address = Address.new(street: "Foo")
     @user = User.new(email: "[email protected]", password: "secret", comments: "my comment")
     @builder = BootstrapForm::FormBuilder.new(:user, @user, self, {})
     @horizontal_builder = BootstrapForm::FormBuilder.new(:user, @user, self,