Display "iid of max_iid" in Issuables' sidebar

app/helpers/issuables_helper.rb

@@ -8,20 +8,16 @@ module IssuablesHelper
     "right-sidebar-#{sidebar_gutter_collapsed? ? 'collapsed' : 'expanded'}"
   end
 
-  def issuable_index(issuable)
-    base_issuable_scope(issuable).where('id < ?', issuable.id).size + 1
-  end
-
   def issuables_count(issuable)
-    base_issuable_scope(issuable).size
+    base_issuable_scope(issuable).maximum(:iid)
   end
 
   def next_issuable_for(issuable)
-    base_issuable_scope(issuable).where('id > ?', issuable.id).last
+    base_issuable_scope(issuable).where('iid > ?', issuable.iid).last
   end
 
   def prev_issuable_for(issuable)
-    base_issuable_scope(issuable).where('id < ?', issuable.id).first
+    base_issuable_scope(issuable).where('iid < ?', issuable.iid).first
   end
 
   private
@@ -31,7 +27,7 @@ module IssuablesHelper
   end
 
   def base_issuable_scope(issuable)
-    issuable.project.send(issuable.to_scope_name).send(issuable_state_scope(issuable))
+    issuable.project.send(issuable.class.table_name).send(issuable_state_scope(issuable))
   end
 
   def issuable_state_scope(issuable)

app/models/concerns/issuable.rb

@@ -168,16 +168,6 @@ module Issuable
     self.class.to_s.underscore
   end
 
-  # Convert this Issuable class name to a format usable for scoping
-  #
-  # Examples:
-  #
-  #   issuable.class         # => MergeRequest
-  #   issuable.to_scope_name # => "merge_requests"
-  def to_scope_name
-    self.class.to_s.tableize
-  end
-
   # Returns a Hash of attributes to be used for Twitter card metadata
   def card_attributes
     {

app/views/shared/issuable/_sidebar.html.haml

@@ -2,7 +2,7 @@
   .issuable-sidebar
     .block
       %span.issuable-count.pull-left
-        = issuable_index(issuable)
+        = issuable.iid
         of
         = issuables_count(issuable)
       %span.pull-right