2 files changed, 30 insertions, 0 deletions
diff --git a/activestorage/app/models/active_storage/blob.rb b/activestorage/app/models/active_storage/blob.rb
index ff785d4f61..d43049e32c 100644
--- a/activestorage/app/models/active_storage/blob.rb
+++ b/activestorage/app/models/active_storage/blob.rb
@@ -15,6 +15,7 @@
 # If you need to create a derivative or otherwise change the blob, simply create a new blob and purge the old one.
 class ActiveStorage::Blob < ActiveRecord::Base
   class UnpreviewableError < StandardError; end
+  class UnrepresentableError < StandardError; end
 
   self.table_name = "active_storage_blobs"
 
@@ -155,6 +156,31 @@ class ActiveStorage::Blob < ActiveRecord::Base
   end
 
 
+  # Returns an ActiveStorage::Preview instance for a previewable blob or an ActiveStorage::Variant instance for an image blob.
+  #
+  #   blob.representation(resize: "100x100").processed.service_url
+  #
+  # Raises ActiveStorage::Blob::UnrepresentableError if the receiving blob is neither an image nor previewable. Call
+  # ActiveStorage::Blob#representable? to determine whether a blob is representable.
+  #
+  # See ActiveStorage::Blob#preview and ActiveStorage::Blob#variant for more information.
+  def representation(transformations)
+    case
+    when previewable?
+      preview transformations
+    when image?
+      variant transformations
+    else
+      raise UnrepresentableError
+    end
+  end
+
+  # Returns true if the blob is an image or is previewable.
+  def representable?
+    image? || previewable?
+  end
+
+
   # Returns the URL of the blob on the service. This URL is intended to be short-lived for security and not used directly
   # with users. Instead, the +service_url+ should only be exposed as a redirect from a stable, possibly authenticated URL.
   # Hiding the +service_url+ behind a redirect also gives you the power to change services without updating all URLs. And
diff --git a/activestorage/app/models/active_storage/variant.rb b/activestorage/app/models/active_storage/variant.rb
index 90a3605331..915b78162c 100644
--- a/activestorage/app/models/active_storage/variant.rb
+++ b/activestorage/app/models/active_storage/variant.rb
@@ -65,6 +65,10 @@ class ActiveStorage::Variant
     service.url key, expires_in: expires_in, disposition: disposition, filename: blob.filename, content_type: blob.content_type
   end
 
+  # Returns the receiving variant. Allows ActiveStorage::Variant and ActiveStorage::Preview instances to be duck-typed.
+  def image
+    self
+  end
 
   private
     def processed?