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

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.

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

more fun:

  validates :password, exclusion: { in: ->(person) { [person.email] }, message: "cannot use protected password" }

which might be even better in this case, since you could check for other forbidden values with something like

  validates :password, exclusion: { in: ->(person) { [person.email, person.first_name, person.last_name, person.phone_number, person.department_name] }, message: "cannot use protected password" }

all you need is to create validation rule in your model for example

class User < ActiveRecord::Base
  def validate_on_create
    if email == password
      errors.add("password", "email and password can't be the same")
    end
  end
end

Ruby on Rails 7.0 added support for validates_comparison_of validations like this

validates :password, comparison: { other_than: :email }

By using custom validations we can do this operation

validate :validate_address

def validate_address
errors.add(:permenent_address, "can't be the same as present_address") if self.present_address== self.permenent_address end