Change ActionText::RichText#embeds assignment to before_validation

Prior to this commit, assignment to the `embeds` association happens
_after_ validation callbacks, so it isn't possible to incorporate Rich
Text-related file validation.

For example, consider rejecting `text/plain` files when creating
`Message` records:

```ruby
class Message < ApplicationRecord
  has_rich_text :content

  validate do
    if content.embeds.any? { |attachment| attachment.blob.content_type == "text/plain" }
      errors.add(:content, "Cannot attach text/plain files")
    end
  end
end
```

Without this change, the `content.embeds` association is empty at the
time the `validate` block is evaluated, since that assignment occurs in
a `before_save` callback on the `ActionText::Content` class.

Alongside this change, the commit corrects the documentation on the
`has_many_attached :embeds` line. The returned collection is of
`ActiveStorage::Attachment` records and **not** `ActiveStorage::Blob`
records as documented.

Author: seanpdoyle

actiontext/CHANGELOG.md

@@ -1,2 +1,5 @@
+*   Change `ActionText::RichText#embeds` assignment from `before_save` to `before_validation`
+
+    *Sean Doyle*
 
 Please check [8-0-stable](https://github.com/rails/rails/blob/8-0-stable/actiontext/CHANGELOG.md) for previous changes.

actiontext/app/models/action_text/rich_text.rb

@@ -48,10 +48,13 @@ class RichText < Record
     ##
     # :method: embeds
     #
-    # Returns the `ActiveStorage::Blob`s of the embedded files.
+    # Returns the `ActiveStorage::Attachment` records from the embedded files.
+    #
+    # Attached `ActiveStorage::Blob` records are extracted from the `body`
+    # in a # [before_validation](/classes/ActiveModel/Validations/Callbacks/ClassMethods.html#method-i-before_validation) callback.
     has_many_attached :embeds
 
-    before_save do
+    before_validation do
       self.embeds = body.attachables.grep(ActiveStorage::Blob).uniq if body.present?
     end
 

actiontext/test/unit/model_test.rb

@@ -61,6 +61,21 @@ class ActionText::ModelTest < ActiveSupport::TestCase
     end
   end
 
+  test "embed extraction occurs before validation" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpeg")
+    content = ActionText::Content.new.append_attachables(blob)
+    message = Message.build(subject: "Greetings", content: content)
+
+    assert_changes -> { message.content.embeds.empty? }, from: true, to: false do
+      message.content.validate
+    end
+
+    embeds = message.content.embeds
+    assert_kind_of ActiveStorage::Attached::Many, embeds
+    assert_kind_of ActiveStorage::Attachment, embeds.first
+    assert_equal blob, embeds.first.blob
+  end
+
   test "saving content" do
     message = Message.create!(subject: "Greetings", content: "<h1>Hello world</h1>")
     assert_equal "Hello world", message.content.to_plain_text