Post#approve!: move validation to post_approval.rb

app/models/post_approval.rb

@@ -1,12 +1,24 @@
 class PostApproval < ActiveRecord::Base
   belongs_to :user
-  belongs_to :post
+  belongs_to :post, inverse_of: :approvals
+
+  validate :validate_approval
 
   def self.prune!
     where("created_at < ?", 1.month.ago).delete_all
   end
 
-  def self.approved?(user_id, post_id)
-    where(user_id: user_id, post_id: post_id).exists?
+  def validate_approval
+    if post.is_status_locked?
+      errors.add(:post, "is locked and cannot be approved")
+    end
+
+    if post.uploader == user
+      errors.add(:base, "You cannot approve a post you uploaded")
+    end
+
+    if post.approved_by?(user)
+      errors.add(:base, "You have previously approved this post and cannot approve it again")
+    end
   end
 end