How do I validate that two values do not equal each other in a Rails model?

Question

Create custom validataion:

validate :check_email_and_password

def check_email_and_password
  errors.add(:password, "can't be the same as email") if email == password
end

But keep in mind that storing password as a plain text is bad idea. You should store it hashed. Try some authentication plugin like authlogic or Restful authentication.

New way:

validates :password, exclusion: { in: lambda{ |user| [user.email] } }

or:

validates :password, exclusion: { in: ->(user) { [user.email] } }

You can use a custom validation method to check this.

class User < ActiveRecord::Base
  # ...

  def validate
    if (self.email == self.password)
      errors.add(:password, "password cannot equal email")
      errors.add(:email, "email cannot equal password")
    end
  end
end

It depends how Your password is stored:

class User < ActiveRecord::Base
    validate :email_and_password_validation

    def email_and_password_validation
        if self.email == self.password
            errors.add_to_base("Password must be different from email") 
        end
    end
end

This would work if Your password is stored literally, but You can perform the same thing with email (e.g. create a hashed version) and check for equality with password. E.g:

class User < ActiveRecord::Base
    validate :email_and_password_validation

    def email_and_password_validation
        if make_hash(self.email) == self.hashed_password
            errors.add_to_base("Password must be different from email") 
        end
    end
end

My example is taken from http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M002162

Your situation is quite general so You can be interested in creating custom validation method. Everything is covered here: http://guides.rubyonrails.org/active_record_validations_callbacks.html#creating-custom-validation-methods