Extend image_tag to accept ActiveStorage Attachments and Variants (#30084)

* Extend image_tag to accept ActiveStorage's Attachments and Variants

* Flip resolve_image_source around

* Add tests for the new use-cases of image_tag

* Remove the higher-level test

* Update image_tag documentation

* Add error states into the test suite

* Re-raise polymorhic_url's NoMethodError as ArgumentError

* delegate_missing_to will raise DelegationError instead of NoMethodError

2 files changed, 41 insertions, 1 deletions

diff --git a/activestorage/README.md b/activestorage/README.md
index aad11325e3..957adc05c3 100644
--- a/activestorage/README.md
+++ b/activestorage/README.md
@@ -80,7 +80,7 @@ Variation of image attachment:
 
 ```erb
 <%# Hitting the variant URL will lazy transform the original blob and then redirect to its new service location %>
-<%= image_tag url_for(user.avatar.variant(resize: "100x100")) %>
+<%= image_tag user.avatar.variant(resize: "100x100") %>
 ```
 
 ## Installation
diff --git a/activestorage/test/template/image_tag_test.rb b/activestorage/test/template/image_tag_test.rb
new file mode 100644
index 0000000000..83c95c01a1
--- /dev/null
+++ b/activestorage/test/template/image_tag_test.rb
@@ -0,0 +1,40 @@
+require "test_helper"
+require "database/setup"
+
+class User < ActiveRecord::Base
+  has_one_attached :avatar
+end
+
+class ActiveStorage::ImageTagTest < ActionView::TestCase
+  tests ActionView::Helpers::AssetTagHelper
+
+  setup do
+    @blob = create_image_blob filename: "racecar.jpg"
+  end
+
+  test "blob" do
+    assert_dom_equal %(<img alt="Racecar" src="#{polymorphic_url @blob}" />), image_tag(@blob)
+  end
+
+  test "variant" do
+    variant = @blob.variant(resize: "100x100")
+    assert_dom_equal %(<img alt="Racecar" src="#{polymorphic_url variant}" />), image_tag(variant)
+  end
+
+  test "attachment" do
+    attachment = ActiveStorage::Attachment.new(blob: @blob)
+    assert_dom_equal %(<img alt="Racecar" src="#{polymorphic_url attachment}" />), image_tag(attachment)
+  end
+
+  test "error when attachment's empty" do
+    @user = User.create!(name: "DHH")
+
+    assert_not @user.avatar.attached?
+    assert_raises(ArgumentError) { image_tag(@user.avatar) }
+  end
+
+  test "error when object can't be resolved into url" do
+    unresolvable_object = ActionView::Helpers::AssetTagHelper
+    assert_raises(ArgumentError) { image_tag(unresolvable_object) }
+  end
+end