Preserve existing attachment assignment behavior for upgraded apps

Assigning to a collection of attachments appends rather than replacing, as in 5.2. Existing 5.2 apps that rely on this behavior will no longer break when they're upgraded to 6.0.

For apps generated on 6.0 or newer, assigning replaces the existing attachments in the collection. #attach should be used to add new attachments to the collection without removing existing ones.

I expect that we'll deprecate the old behavior in 6.1.

Closes #36374.

3 files changed, 58 insertions, 5 deletions

diff --git a/guides/source/6_0_release_notes.md b/guides/source/6_0_release_notes.md
index c152076628..7c5478a03d 100644
--- a/guides/source/6_0_release_notes.md
+++ b/guides/source/6_0_release_notes.md
@@ -672,6 +672,12 @@ Please refer to the [Changelog][active-storage] for detailed changes.
     is saved instead of immediately.
     ([Pull Request](https://github.com/rails/rails/pull/33303))
 
+*   Optionally replace existing files instead of adding to them when assigning to
+    a collection of attachments (as in `@user.update!(images: [ … ])`). Use
+    `config.active_storage.replace_on_assign_to_many` to control this behavior.
+    ([Pull Request](https://github.com/rails/rails/pull/33303),
+     [Pull Request](https://github.com/rails/rails/pull/36716))
+
 *   Add the ability to reflect on defined attachments using the existing
     Active Record reflection mechanism.
     ([Pull Request](https://github.com/rails/rails/pull/33018))
@@ -688,10 +694,6 @@ Please refer to the [Changelog][active-storage] for detailed changes.
     `mini_magick` directly.
     ([Pull Request](https://github.com/rails/rails/pull/32471))
 
-*   Replace existing images instead of adding to them when updating an
-    attached model via `update` or `update!` with, say, `@user.update!(images: [ … ])`.
-    ([Pull Request](https://github.com/rails/rails/pull/33303))
-
 Active Model
 ------------
 
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index e53e8b4e92..4e0224b61e 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -881,7 +881,9 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
   config.active_storage.routes_prefix = '/files'
   ```
 
-  The default is `/rails/active_storage`
+  The default is `/rails/active_storage`.
+
+* `config.active_storage.replace_on_assign_to_many` determines whether assigning to a collection of attachments declared with `has_many_attached` replaces any existing attachments or appends to them. The default is `true`.
 
 ### Results of `load_defaults`
 
@@ -917,6 +919,7 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
 - `config.active_job.return_false_on_aborted_enqueue`: `true`
 - `config.active_storage.queues.analysis`: `:active_storage_analysis`
 - `config.active_storage.queues.purge`: `:active_storage_purge`
+- `config.active_storage.replace_on_assign_to_many`: `true`
 - `config.active_record.collection_cache_versioning`: `true`
 
 ### Configuring a Database
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index f17955e022..05980d1614 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -398,6 +398,54 @@ config.load_defaults "6.0"
 config.autoloader = :classic
 ```
 
+### Active Storage assignment behavior change
+
+In Rails 5.2, assigning to a collection of attachments declared with `has_many_attached` appended new files:
+
+```ruby
+class User < ApplicationRecord
+  has_many_attached :highlights
+end
+
+user.highlights.attach(filename: "funky.jpg", ...)
+user.higlights.count # => 1
+
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.update!(highlights: [ blob ])
+
+user.highlights.count # => 2
+user.highlights.first.filename # => "funky.jpg"
+user.highlights.second.filename # => "town.jpg"
+```
+
+With the default configuration for Rails 6.0, assigning to a collection of attachments replaces existing files
+instead of appending to them. This matches Active Record behavior when assigning to a collection association:
+
+```ruby
+user.highlights.attach(filename: "funky.jpg", ...)
+user.highlights.count # => 1
+
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.update!(highlights: [ blob ])
+
+user.highlights.count # => 1
+user.highlights.first.filename # => "town.jpg"
+```
+
+`#attach` can be used to add new attachments without removing the existing ones:
+
+```ruby
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.highlights.attach(blob)
+
+user.highlights.count # => 2
+user.highlights.first.filename # => "funky.jpg"
+user.highlights.second.filename # => "town.jpg"
+```
+
+Opt in to the new default behavior by setting `config.active_storage.replace_on_assign_to_many` to `true`.
+The old behavior will be deprecated in Rails 6.1 and removed in a subsequent release.
+
 Upgrading from Rails 5.1 to Rails 5.2
 -------------------------------------