Original implementation to allow users to subscribe to labels

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

2 files changed, 50 insertions, 23 deletions

diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 3c42f582937..affc4a842a7 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -8,6 +8,7 @@ module Issuable
   extend ActiveSupport::Concern
   include Participable
   include Mentionable
+  include Subscribable
   include StripAttribute
 
   included do
@@ -18,7 +19,6 @@ module Issuable
     has_many :notes, as: :noteable, dependent: :destroy
     has_many :label_links, as: :target, dependent: :destroy
     has_many :labels, through: :label_links
-    has_many :subscriptions, dependent: :destroy, as: :subscribable
 
     validates :author, presence: true
     validates :title, presence: true, length: { within: 0..255 }
@@ -149,28 +149,6 @@ module Issuable
     notes.awards.where(note: "thumbsup").count
   end
 
-  def subscribed?(user)
-    subscription = subscriptions.find_by_user_id(user.id)
-
-    if subscription
-      return subscription.subscribed
-    end
-
-    participants(user).include?(user)
-  end
-
-  def toggle_subscription(user)
-    subscriptions.
-      find_or_initialize_by(user_id: user.id).
-      update(subscribed: !subscribed?(user))
-  end
-
-  def unsubscribe(user)
-    subscriptions.
-      find_or_initialize_by(user_id: user.id).
-      update(subscribed: false)
-  end
-
   def to_hook_data(user)
     hook_data = {
       object_kind: self.class.name.underscore,
@@ -201,6 +179,12 @@ module Issuable
     end
   end
 
+  # Labels that are currently applied to this object
+  # that are not present in `old_labels`
+  def added_labels(old_labels)
+    self.labels - old_labels
+  end
+
   # Convert this Issuable class name to a format usable by Ability definitions
   #
   # Examples:
diff --git a/app/models/concerns/subscribable.rb b/app/models/concerns/subscribable.rb
new file mode 100644
index 00000000000..cab9241ac3d
--- /dev/null
+++ b/app/models/concerns/subscribable.rb
@@ -0,0 +1,43 @@
+# == Subscribable concern
+#
+# Users can subscribe to these models.
+#
+# Used by Issue, MergeRequest, Label
+#
+
+module Subscribable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :subscriptions, dependent: :destroy, as: :subscribable
+  end
+
+  def subscribed?(user)
+    subscription = subscriptions.find_by_user_id(user.id)
+
+    if subscription
+      return subscription.subscribed
+    end
+
+    # FIXME
+    # Issue/MergeRequest has participants, but Label doesn't.
+    # Ideally, subscriptions should be separate from participations,
+    # but that seems like a larger change with farther-reaching
+    # consequences, so this is a compromise for the time being.
+    if respond_to?(:participants)
+      participants(user).include?(user)
+    end
+  end
+
+  def toggle_subscription(user)
+    subscriptions.
+      find_or_initialize_by(user_id: user.id).
+      update(subscribed: !subscribed?(user))
+  end
+
+  def unsubscribe(user)
+    subscriptions.
+      find_or_initialize_by(user_id: user.id).
+      update(subscribed: false)
+  end
+end