[10] pry(main)> r.respondents.select(:name).uniq.size
(1.1ms) SELECT DISTINCT COUNT("respondents"."name") FROM "respondents"
INNER JOIN "values" ON "respondents"."id" = "values"."respondent_id" WHERE
"values"."round_id" = 37 => 495
[11] pry(main)> r.respondents.select(:name).uniq.length
Respondent Load (1.1ms) SELECT DISTINCT name FROM "respondents"
INNER JOIN "values" ON "respondents"."id" = "values"."respondent_id" WHERE
"values"."round_id" = 37 => 6
为什么每个查询返回的内容不同？
最佳答案
.count #=> this always triggers a SELECT COUNT(*) on the database
.size #=> if the collection has been loaded, defers to Enumerable#size, else does the SELECT COUNT(*)
.length #=> always loads the collection and then defers to Enumerable#size
关于sql - 复杂的 ActiveRecord 案例中的大小、长度和计数之间的差异，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/11905364/