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.

9 files changed, 117 insertions, 15 deletions

diff --git a/activestorage/lib/active_storage.rb b/activestorage/lib/active_storage.rb
index 5c5da551ae..75eb63f0b4 100644
--- a/activestorage/lib/active_storage.rb
+++ b/activestorage/lib/active_storage.rb
@@ -43,18 +43,26 @@ module ActiveStorage
 
   mattr_accessor :logger
   mattr_accessor :verifier
+  mattr_accessor :variant_processor, default: :mini_magick
+
   mattr_accessor :queues, default: {}
+
   mattr_accessor :previewers, default: []
-  mattr_accessor :analyzers, default: []
-  mattr_accessor :variant_processor, default: :mini_magick
+  mattr_accessor :analyzers,  default: []
+
   mattr_accessor :paths, default: {}
-  mattr_accessor :variable_content_types, default: []
+
+  mattr_accessor :variable_content_types,           default: []
+  mattr_accessor :binary_content_type,              default: "application/octet-stream"
   mattr_accessor :content_types_to_serve_as_binary, default: []
-  mattr_accessor :content_types_allowed_inline, default: []
-  mattr_accessor :binary_content_type, default: "application/octet-stream"
+  mattr_accessor :content_types_allowed_inline,     default: []
+
   mattr_accessor :service_urls_expire_in, default: 5.minutes
+
   mattr_accessor :routes_prefix, default: "/rails/active_storage"
 
+  mattr_accessor :replace_on_assign_to_many, default: false
+
   module Transformers
     extend ActiveSupport::Autoload
 
diff --git a/activestorage/lib/active_storage/attached/model.rb b/activestorage/lib/active_storage/attached/model.rb
index ae7f0685f2..06864a846f 100644
--- a/activestorage/lib/active_storage/attached/model.rb
+++ b/activestorage/lib/active_storage/attached/model.rb
@@ -93,12 +93,19 @@ module ActiveStorage
           end
 
           def #{name}=(attachables)
-            attachment_changes["#{name}"] =
-              if attachables.nil? || Array(attachables).none?
-                ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
-              else
-                ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+            if ActiveStorage.replace_on_assign_to_many
+              attachment_changes["#{name}"] =
+                if attachables.nil? || Array(attachables).none?
+                  ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
+                else
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+                end
+            else
+              if !attachables.nil? || Array(attachables).any?
+                attachment_changes["#{name}"] =
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, #{name}.blobs + attachables)
               end
+            end
           end
         CODE
 
diff --git a/activestorage/lib/active_storage/engine.rb b/activestorage/lib/active_storage/engine.rb
index f70d0a512a..e88e7fa14e 100644
--- a/activestorage/lib/active_storage/engine.rb
+++ b/activestorage/lib/active_storage/engine.rb
@@ -79,6 +79,8 @@ module ActiveStorage
         ActiveStorage.service_urls_expire_in = app.config.active_storage.service_urls_expire_in || 5.minutes
         ActiveStorage.content_types_allowed_inline = app.config.active_storage.content_types_allowed_inline || []
         ActiveStorage.binary_content_type = app.config.active_storage.binary_content_type || "application/octet-stream"
+
+        ActiveStorage.replace_on_assign_to_many = app.config.active_storage.replace_on_assign_to_many || false
       end
     end
 
diff --git a/activestorage/test/models/attached/many_test.rb b/activestorage/test/models/attached/many_test.rb
index 878e284049..39ddecb041 100644
--- a/activestorage/test/models/attached/many_test.rb
+++ b/activestorage/test/models/attached/many_test.rb
@@ -269,6 +269,24 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     end
   end
 
+  test "updating an existing record with attachments when appending on assign" do
+    append_on_assign do
+      @user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
+
+      assert_difference -> { @user.reload.highlights.count }, +2 do
+        @user.update! highlights: [ create_blob(filename: "whenever.jpg"), create_blob(filename: "wherever.jpg") ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: [ ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: nil
+      end
+    end
+  end
+
   test "attaching existing blobs to a new record" do
     User.new(name: "Jason").tap do |user|
       user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
@@ -538,4 +556,12 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
       User.remove_method :highlights
     end
   end
+
+  private
+    def append_on_assign
+      ActiveStorage.replace_on_assign_to_many, previous = false, ActiveStorage.replace_on_assign_to_many
+      yield
+    ensure
+      ActiveStorage.replace_on_assign_to_many = previous
+    end
 end
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
 -------------------------------------
 
diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index 50d43ff69e..934578e9f1 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -145,6 +145,8 @@ module Rails
           if respond_to?(:active_storage)
             active_storage.queues.analysis = :active_storage_analysis
             active_storage.queues.purge    = :active_storage_purge
+
+            active_storage.replace_on_assign_to_many = true
           end
 
           if respond_to?(:active_record)
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
index abb03e761b..ffe53497bf 100644
--- a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
@@ -26,6 +26,10 @@
 # Rails.application.config.active_storage.queues.analysis = :active_storage_analysis
 # Rails.application.config.active_storage.queues.purge    = :active_storage_purge
 
+# When assigning to a collection of attachments declared via `has_many_attached`, replace existing
+# attachments instead of appending. Use #attach to add new attachments without replacing existing ones.
+# Rails.application.config.active_storage.replace_on_assign_to_many = true
+
 # Use ActionMailer::MailDeliveryJob for sending parameterized and normal mail.
 #
 # The default delivery jobs (ActionMailer::Parameterized::DeliveryJob, ActionMailer::DeliveryJob),