Rails 中的大小、长度和数量 [英] size, length and count in Rails
问题描述
1.9.3p194 :002 > u = User.find_by_email("email@mail.ru")
1.9.3p194 :005 > u.addresses.size
=> 1
1.9.3p194 :006 > u.addresses.length
=> 1
1.9.3p194 :007 > u.addresses.count
Rails 3.2.3 中的大小、长度和数量没有区别，不是吗?
There is no difference between size, length and count in Rails 3.2.3, isn't it?
推荐答案
length 将加载您所有的对象只是为了计算它们；类似:
length will load all your objects just to count them; something like:
select * from addresses...
然后返回结果计数.你可以想象 - 这是糟糕的表现
and then return the results count. As you can imagine - it's bad performance
count 只会发出
select count(*) from addresses...
哪个更好，因为我们不加载所有地址只是为了计算它们
which is better, because we are not loading all addresses just to count them
size 更智能 - 它会检查关联是否已加载，如果为 true，则返回长度(不调用数据库).
size is smarter - it'll check if the association is already loaded and if true then return the length (without issuing a call to the database).
size 还会检查 counter_cache 如果您的用户模型中有一个名为 address_count 的字段，则 size 将使用此字段进行计数，因此无需对地址表进行计数.
size also checks for counter_cache if you have a field named address_count in your user model, then size will use this field for the count, so there is no need to issue a count on the addresses table.
如果都失败了，size会在数据库上发出一个select count(*)
if all fails, size will issue a select count(*) on the database
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