Create Participants for new Members

app/models/group.rb

     self.members.includes(:person).where(is_leader: true)
   end
 
+  # @return [Array<Activity>] the activities that haven't started yet.
+  def future_activities
+    self.activities.where('start > ?', DateTime.now)
+  end
+
   # Determine whether the passed person is a group leader.
   def is_leader?(person)
     Member.exists?(

app/models/member.rb

   belongs_to :person
   belongs_to :group
 
+  after_create   :create_future_participants!
   before_destroy :delete_future_participants!
 
   validates :person_id,
       message: "is already a member of this group"
     }
 
+  # Create Participants for this Member for all the group's future activities, where the member isn't enrolled yet.
+  # Intended to be called after the member is added to the group.
+  def create_future_participants!
+    activities = self.group.future_activities
+
+    if not self.person.activities.empty?
+      activities = activities.where(
+        'activities.id NOT IN (?)', self.person.activities.ids
+      )
+    end
+
+    activities.each do |a|
+      Participant.create!(
+        activity: a,
+        person: self.person
+      )
+    end
+  end
+
   # Delete all Participants of this Member for Activities in the future.
   # Intended to be called before the member is deleted.
   def delete_future_participants!
-    activities = self.group.activities
-      .where('start > ?', DateTime.now)
-
     participants = Participant.where(
       person_id: self.person.id,
-      activity: activities
+      activity: self.group.future_activities
     )
 
     participants.each do |p|