6 files changed, 63 insertions, 21 deletions
diff --git a/guides/source/5_2_release_notes.md b/guides/source/5_2_release_notes.md
index ac247bc3f9..7aac07dbbe 100644
--- a/guides/source/5_2_release_notes.md
+++ b/guides/source/5_2_release_notes.md
@@ -326,7 +326,7 @@ Please refer to the [Changelog][action-view] for detailed changes.
     select divider `option`.
     ([Pull Request](https://github.com/rails/rails/pull/31088))
 
-*   Change `form_with` to generates ids by default.
+*   Change `form_with` to generate ids by default.
     ([Commit](https://github.com/rails/rails/commit/260d6f112a0ffdbe03e6f5051504cb441c1e94cd))
 
 *   Add `preload_link_tag` helper.
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/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 8cb49ca6ae..f36cacfe8d 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -155,15 +155,6 @@ Complex(1).duplicable?      # => true
 1.method(:+).duplicable?    # => false
 ```
 
-`duplicable?` matches the current Ruby version's `dup` behavior,
-so results will vary according the version of Ruby you're using.
-In Ruby 2.4, for example, Complex and Rational are not duplicable:
-
-```ruby
-Rational(1).duplicable?     # => false
-Complex(1).duplicable?      # => false
-```
-
 WARNING: Any class can disallow duplication by removing `dup` and `clone` or raising exceptions from them. Thus only `rescue` can tell whether a given arbitrary object is duplicable. `duplicable?` depends on the hard-coded list above, but it is much faster than `rescue`. Use it only if you know the hard-coded list is enough in your use case.
 
 NOTE: Defined in `active_support/core_ext/object/duplicable.rb`.
@@ -2358,10 +2349,6 @@ There's also a related idiom that uses the splat operator:
 [*object]
 ```
 
-which in Ruby 1.8 returns `[nil]` for `nil`, and calls to `Array(object)` otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)
-
-Thus, in this case the behavior is different for `nil`, and the differences with `Kernel#Array` explained above apply to the rest of `object`s.
-
 NOTE: Defined in `active_support/core_ext/array/wrap.rb`.
 
 ### Duplicating
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index e53e8b4e92..ded985debe 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -353,7 +353,7 @@ All these configuration options are delegated to the `I18n` library.
 
 * `config.active_record.lock_optimistically` controls whether Active Record will use optimistic locking and is `true` by default.
 
-* `config.active_record.cache_timestamp_format` controls the format of the timestamp value in the cache key. Default is `:nsec`.
+* `config.active_record.cache_timestamp_format` controls the format of the timestamp value in the cache key. Default is `:usec`.
 
 * `config.active_record.record_timestamps` is a boolean value which controls whether or not timestamping of `create` and `update` operations on a model occur. The default value is `true`.
 
@@ -881,7 +881,11 @@ 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`.
+
+* `config.active_storage.draw_routes` can be used to toggle Active Storage route generation. The default is `true`.
 
 ### Results of `load_defaults`
 
@@ -917,6 +921,7 @@ 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/engines.md b/guides/source/engines.md
index 90b08b00f0..8961a079b5 100644
--- a/guides/source/engines.md
+++ b/guides/source/engines.md
@@ -220,7 +220,7 @@ Inside the `app` directory are the standard `assets`, `controllers`, `helpers`,
 `jobs`, `mailers`, `models`, and `views` directories that you should be familiar with
 from an application. We'll look more into models in a future section, when we're writing the engine.
 
-Within the `app/assets` directory, there are the `images`, `javascripts` and
+Within the `app/assets` directory, there are the `images` and
 `stylesheets` directories which, again, you should be familiar with due to their
 similarity to an application. One difference here, however, is that each
 directory contains a sub-directory with the engine name. Because this engine is
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
 -------------------------------------