Added ransack search function to admin/users index

app/models/user.rb

@@ -42,4 +42,16 @@ class User < ApplicationRecord
   def name
     [ firstname, " ", lastname ].join
   end
+
+  def self.ransackable_attributes(auth_object = nil)
+    [ "created_at", "email", "firstname", "id",  "jobs_as_costumer_count", "jobs_as_operator_count", "lastname", "role", "verified", "name" ]
+  end
+
+  def self.ransackable_associations(auth_object = nil)
+    []
+  end
+
+  ransacker :name do
+    Arel.sql("CONCAT_WS(' ', users.firstname, users.lastname)")
+  end
 end