什么是在ActiveRecord中计算子对象数量的最佳方法

Question

class Foo < ActiveRecord::Base {
   has_many :bars
}

foo = Foo.find(1)
foo.bars.count

Is this the best way to count the number of bars or is there an efficient way

Answer 1

If your model is completely normalized then this will be the most efficent way.

If you need to do this a lot, you can cache the bars count on the Foo table.

Railscast Episode 23 covers counter cache, this information is still correct an relevant in the current Rails version.

Answer 2

是的，这是最有效的方法，因为ActiveRecord将其转换为适当的SELECT COUNT（*）SQL语句，这比加载所有记录数据并将它们计数到数组中要高效得多。