application record: drop execute_sql, select_value_sql methods.

app/models/post_vote.rb

@@ -11,16 +11,19 @@ class PostVote < ApplicationRecord
   after_create :update_post_on_create
   after_destroy :update_post_on_destroy
 
+  scope :positive, -> { where("post_votes.score > 0") }
+  scope :negative, -> { where("post_votes.score < 0") }
+
   def self.positive_user_ids
-    select_values_sql("select user_id from post_votes where score > 0 group by user_id having count(*) > 100")
+    positive.group(:user_id).having("count(*) > 100").pluck(:user_id)
   end
 
   def self.negative_post_ids(user_id)
-    select_values_sql("select post_id from post_votes where score < 0 and user_id = ?", user_id)
+    negative.where(user_id: user_id).pluck(:post_id)
   end
 
   def self.positive_post_ids(user_id)
-    select_values_sql("select post_id from post_votes where score > 0 and user_id = ?", user_id)
+    positive.where(user_id: user_id).pluck(:post_id)
   end
 
   def self.visible(user = CurrentUser.user)