Sett iid on create. Use iid as value to_param

app/models/concerns/issuable.rb

@@ -16,6 +16,7 @@ module Issuable
 
     validates :author, presence: true
     validates :title, presence: true, length: { within: 0..255 }
+    validates :iid, presence: true, numericality: true
 
     scope :authored, ->(user) { where(author_id: user) }
     scope :assigned_to, ->(u) { where(assignee_id: u.id)}
@@ -24,6 +25,8 @@ module Issuable
     scope :unassigned, -> { where("assignee_id IS NULL") }
     scope :of_projects, ->(ids) { where(project_id: ids) }
 
+    validate :set_iid, on: :create
+
     delegate :name,
              :email,
              to: :author,
@@ -44,6 +47,15 @@ module Issuable
     end
   end
 
+  def set_iid
+    max_iid = project.send(self.class.name.tableize).maximum(:iid)
+    self.iid = max_iid.to_i + 1
+  end
+
+  def to_param
+    iid.to_s
+  end
+
   def today?
     Date.today == created_at.to_date
   end

app/models/merge_request.rb

@@ -250,6 +250,10 @@ class MergeRequest < ActiveRecord::Base
     (source_project.root_ref? source_branch) || for_fork?
   end
 
+  def project
+    target_project
+  end
+
   private
 
   def dump_commits(commits)