Refactor select().map() to use pluck.

Remove a method already provided by ActiveRecord.

app/models/category.rb

@@ -112,7 +112,7 @@ class Category < ActiveRecord::Base
   def group_names=(names)
     # this line bothers me, destroying in AR can not seem to be queued, thinking of extending it
     category_groups.destroy_all unless new_record?
-    ids = Group.where(name: names.split(",")).select(:id).map(&:id)
+    ids = Group.where(name: names.split(",")).pluck(:id)
     ids.each do |id|
       category_groups.build(group_id: id)
     end

app/models/group.rb

@@ -129,11 +129,7 @@ class Group < ActiveRecord::Base
   end
 
   def usernames
-    users.select("username").map(&:username).join(",")
+    users.pluck(:username).join(",")
-  end
-
-  def user_ids
-    users.select('users.id').map(&:id)
   end
 
   def add(user)

app/models/post_action.rb

@@ -26,7 +26,7 @@ class PostAction < ActiveRecord::Base
                                     .count('DISTINCT posts.id')
 
     $redis.set('posts_flagged_count', posts_flagged_count)
-    user_ids = User.staff.select(:id).map {|u| u.id}
+    user_ids = User.staff.pluck(:id)
     MessageBus.publish('/flagged_counts', { total: posts_flagged_count }, { user_ids: user_ids })
   end
 

lib/post_destroyer.rb

@@ -43,7 +43,7 @@ class PostDestroyer
       @post.update_flagged_posts_count
 
       # Remove any reply records that point to deleted posts
-      post_ids = PostReply.select(:post_id).where(reply_id: @post.id).map(&:post_id)
+      post_ids = PostReply.where(reply_id: @post.id).pluck(:post_id)
       PostReply.delete_all reply_id: @post.id
 
       if post_ids.present?