Set various many-to-many models to destroy

app/models/activity.rb

 
   belongs_to :group
 
-  has_many :participants
+  has_many :participants,
+    dependent: :destroy
   has_many :people, through: :participants
 
   validates :public_name, presence: true

app/models/group.rb

   #   @return [String]
   #     the name of the group. Must be unique across all groups.
 
-  has_many :members
+  has_many :members,
+    dependent: :destroy
   has_many :people, through: :members
 
-  has_many :activities
+  has_many :activities,
+    dependent: :destroy
 
   validates :name,
     presence: true,

app/models/person.rb

   #     whether or not the person has administrative rights.
 
   has_one :user
-  has_many :members
-  has_many :participants
+  has_many :members,
+    dependent: :destroy
+  has_many :participants,
+    dependent: :destroy
   has_many :groups, through: :members
   has_many :activities, through: :participants