Rails - Why is my custom validation being triggered for only a build command -

i have sentence , correction model has_one , belongs_to relationship respectively.

for reason when do

 def create          @sentence   = sentence.find(params[:sentence_id])         @correction = @sentence.build_correction(params[:correction]) 

a custom validation wrote correction being called @ build_correction point. validation below

class correction < activerecord::base    attr_accessible :text, :sentence_id, :user_id    belongs_to :sentence    belongs_to :user     validate :correction_is_different_than_sentence     def correction_is_different_than_sentence      errors.add(:text, "can't same original sentence.") if (text == self.sentence.text)    end 

the problem reason on validation correction object doesn't have sentence id set (despite used build_correction method) , complains "you have nil object .... while executing nil.text" in if clause in validation above.

so question why validation occuring build command, thought triggers on create or update. , why isnt sentence_id getting set?

some error creating lot of headaches me. don't know why moving custom validator call end of other validator calls fixed me.

before

  validates :name, :short_description, presence: true   validate :uniq_name   validates :price, :numericality => {:greater_than_or_equal_to => 0}   validates_attachment_content_type :image, :content_type => /image/ 

after

  validates :name, :short_description, presence: true   validates :price, :numericality => {:greater_than_or_equal_to => 0}   validates_attachment_content_type :image, :content_type => /image/   validate :uniq_name 

here custom validator

private  def uniq_name   return if clone?   user_product = self.user.products.unlocked.where(:name => self.name).first   errors[:name] << "has been taken" if user_product && !user_product.id.eql?(self.id) end 

try this, may trick too.