validates_presence_of

Validates that the specified attributes are not blank (as defined by Object#blank?), and, if the attribute is an association, that the associated object is not marked for destruction. Happens by default on save.

class Person < ActiveRecord::Base
  has_one :face
  validates_presence_of :face
end

The face attribute must be in the object and it cannot be blank or marked for destruction.

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.

This validator defers to the ActiveModel validation for presence, adding the check to see that an associated object is not marked for destruction. This prevents the parent object from validating successfully and saving, which then deletes the associated object, thus putting the parent object into an invalid state.

Configuration options:

• :message - A custom error message (default is: “can’t be blank”).

• :on - Specifies the contexts where this validation is active. Runs in all validation contexts by default (nil). You can pass a symbol or an array of symbols. (e.g. on: :create or on: :custom_validation_context or on: [:create, :custom_validation_context])

• :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.

• :strict - Specifies whether validation should be strict. See ActiveModel::Validation#validates! for more information.