validation - rails how to validatie data params in method dose not updated, created -

i want validate 1 params in model method, can't found fit answers , please show me right way.

class user < activerecord::base     validate :username, presence: true, length: 4..5, unique: true     validate :email, presence: true, unique: true, format: {with: /\a[a-z0-9\.]+@([a-z]{1,10}\.){1,2}[a-z]{2,4}\z/}     def self.get_post(id)         # how call validate id ???          validates :id, numericality: true         if id.valid?             # true code         else             # false code         end     end     def change_profile         # how check validate user , email         username.valid?         email.valid?          # some_code....      end end 

thanks all.

you cannot use validates there, can instead

def self.get_post(id)         if id.is_a? numeric       # true code   else       # false code   end end