GRN2-xx: Made role priority unique scoped to provider (#942)

* Made role priority unique scoped to provider

* Fixed issues related to update_role after making role priority unique

db/migrate/20200130144841_change_role_priority_to_unique.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class MigrationProduct < ActiveRecord::Base
+  self.table_name = :roles
+end
+
+class ChangeRolePriorityToUnique < ActiveRecord::Migration[5.2]
+  def change
+    reversible do |dir|
+      dir.up do
+        MigrationProduct.where("priority < 0").where.not(name: "pending").each do |role|
+          role.decrement!(:priority)
+        end
+
+        add_index MigrationProduct, [:priority, :provider], unique: true
+      end
+
+      dir.down do
+        remove_index MigrationProduct, [:priority, :provider]
+
+        MigrationProduct.where("priority < 0").where.not(name: "pending").each do |role|
+          role.increment!(:priority)
+        end
+      end
+    end
+  end
+end