Validates that the specified attributes are not blank (as defined by Object#blank?). Happens by default on save. Example:

class Person < ActiveRecord::Base
  validates_presence_of :first_name
end

The first_name attribute must be in the object and it cannot be blank.

If you want to validate the presence of a boolean field (where the real values are true and false), you will want to use validates_inclusion_of :field_name, :in => [true, false] This is due to the way Object#blank? handles boolean values. false.blank? # => true

Configuration options:

• message - A custom error message (default is: "can’t be blank")
• on - Specifies when this validation is active (default is :save, other options :create, :update)
• if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
• unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.

Warning

Validate the presence of the foreign key, not the instance variable itself. Do this:

validates_presence_of :invoice_id

Not this:

validates_presence_of :invoice

If you validate the presence of the associated object, you will get failures on saves when both the parent object and the child object are new.

Source Code

# File active_record/validations.rb, line 512
def validates_presence_of(*attr_names)
  configuration = { :message => ActiveRecord::Errors.default_error_messages[:blank], :on => :save }
  configuration.update(attr_names.extract_options!)

  # can't use validates_each here, because it cannot cope with nonexistent attributes,
  # while errors.add_on_empty can
  send(validation_method(configuration[:on])) do |record|
    unless (configuration[:if] && !evaluate_condition(configuration[:if], record)) || (configuration[:unless] && evaluate_condition(configuration[:unless], record))
      record.errors.add_on_blank(attr_names, configuration[:message])
    end
  end
end