FEATURE: set notification levels when added to a group (#10378)

* FEATURE: set notification levels when added to a group

This feature allows admins and group owners to define default
category and tag tracking levels that will be applied to user
preferences automatically at the time when users are added to the
group. Users are free to change those preferences afterwards.
When removed from a group, the user's notification preferences aren't
changed.

app/models/group.rb

@@ -29,6 +29,8 @@ class Group < ActiveRecord::Base
   has_many :group_histories, dependent: :destroy
   has_many :category_reviews, class_name: 'Category', foreign_key: :reviewable_by_group_id, dependent: :nullify
   has_many :reviewables, foreign_key: :reviewable_by_group_id, dependent: :nullify
+  has_many :group_category_notification_defaults, dependent: :destroy
+  has_many :group_tag_notification_defaults, dependent: :destroy
 
   belongs_to :flair_upload, class_name: 'Upload'
 
@@ -51,6 +53,7 @@ class Group < ActiveRecord::Base
   after_commit :trigger_group_created_event, on: :create
   after_commit :trigger_group_updated_event, on: :update
   after_commit :trigger_group_destroyed_event, on: :destroy
+  after_commit :set_default_notifications, on: [:create, :update]
 
   def expire_cache
     ApplicationSerializer.expire_cache_fragment!("group_names")
@@ -382,6 +385,13 @@ class Group < ActiveRecord::Base
     end
   end
 
+  def self.set_category_and_tag_default_notification_levels!(user, group_name)
+    if group = lookup_group(group_name)
+      GroupUser.set_category_notifications(group, user)
+      GroupUser.set_tag_notifications(group, user)
+    end
+  end
+
   def self.refresh_automatic_group!(name)
     return unless id = AUTO_GROUPS[name]
 
@@ -755,6 +765,32 @@ class Group < ActiveRecord::Base
     flair_icon.presence || flair_upload&.short_path
   end
 
+  [:muted, :tracking, :watching, :watching_first_post].each do |level|
+    define_method("#{level}_category_ids=") do |category_ids|
+      @category_notifications ||= {}
+      @category_notifications[level] = category_ids
+    end
+
+    define_method("#{level}_tags=") do |tag_names|
+      @tag_notifications ||= {}
+      @tag_notifications[level] = tag_names
+    end
+  end
+
+  def set_default_notifications
+    if @category_notifications
+      @category_notifications.each do |level, category_ids|
+        GroupCategoryNotificationDefault.batch_set(self, level, category_ids)
+      end
+    end
+
+    if @tag_notifications
+      @tag_notifications.each do |level, tag_names|
+        GroupTagNotificationDefault.batch_set(self, level, tag_names)
+      end
+    end
+  end
+
   def imap_mailboxes
     return [] if self.imap_server.blank? ||
                  self.email_username.blank? ||