article.rb

class Article < ActiveRecord::Base
  default_scope where(:published => true)
end

sql.log

> Article.all
  SQL ->  SELECT "articles".* 
    FROM "articles" WHERE "articles"."published" = 't'

> Article.where(title: "a")
  SQL ->  SELECT "articles".* 
    FROM "articles" WHERE "articles"."published" = 't' 
    AND "articles"."title" = 'a'

> Article.where(published:false)
  SQL ->  SELECT "articles".* 
    FROM "articles" WHERE "articles"."published" = 'f

> Article.where(published:nil)
  SQL ->  SELECT "articles".* 
    FROM "articles" WHERE "articles"."published" IS NULL

> Article.instance_eval{
>   with_exclusive_scope{
>    Article.where(title: "a")
>   }
> }
  SQL ->  SELECT "articles".* 
    FROM "articles" WHERE "articles"."title" = 'a'