Rails重构：更有效的ActiveRecord查询和条件测试

Question

In a model I am trying to return a conditional ActiveRecord result set. If the author has written books, return those. If the author hasn't written any books, return articles. This works:

def writings  
  Books.where(author_id: 1).present? ? Books.where(author_id: 1)  : Articles.where(author_id: 2)  
end

How can this be improved
1. When checking conditions and setting values, ie not executing the Books.where query twice?
2. From a performance standpoint

I'm working on improving my efficiency and style and often use either Style Guide bbatsov or the addition .

Answer 1

You can use the presence method combined with a double pipe:

def writings
  Books.where(author_id: 1).presence || Articles.where(author_id: 2)
end

---

From the documentation: http://apidock.com/rails/Object/presence

presence() public

Returns the receiver if it's present otherwise returns nil .

Some examples:

true.presence # => true
1.presence # => 1
false.presence # => nil
[].presence # => nil
''.presence # => nil
User.where(id: -1).presence # => nil

Answer 2

Consider implementing counter_cache on both Book and Article models.

# class Book
belongs_to :author, counter_cache: true

# class Article
belongs_to :author, counter_cache: true

Read more about counter caching here .

Then you can inspect the value of books_count before running any queries.

# class Author
# assumes has_many :books
# and     has_many :articles
def writings
  if books_count.nonzero?
    books
  else
    articles
  end
end

Answer 3

Well, first of all you are now always doing 2 queries. 1 - to get present?, second to get either books or Articles.

You can improve by changing code so that is selects books and if there are none, then returns articles.

def writings  
  books = Books.where(author_id: 1) 
  if books.size > 0 
    books
  else
    Articles.where(author_id: 2)  
  end
end