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.

activestorage/lib/active_storage.rb

@@ -43,18 +43,26 @@ module ActiveStorage
 
   mattr_accessor :logger
   mattr_accessor :verifier
-  mattr_accessor :queues, default: {}
-  mattr_accessor :previewers, default: []
-  mattr_accessor :analyzers, default: []
   mattr_accessor :variant_processor, default: :mini_magick
+
+  mattr_accessor :queues, default: {}
+
+  mattr_accessor :previewers, default: []
+  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
 

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
 

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
 

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

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
 ------------
 

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

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
 -------------------------------------
 

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)

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),