sql - 使用Rails Console SQL查询字符串学习SQL

Question

使用ActiveRecord内部的ORM rails console，我可以轻松而合理地获得：第一件商品的款式零售价：

[25] pry(main)> Item.first.style.retail_price
  Item Load (0.2ms)  SELECT  "items".* FROM "items"  ORDER BY "items"."id" ASC LIMIT 1
  Style Load (0.1ms)  SELECT  "styles".* FROM "styles" WHERE "styles"."id" = ? LIMIT 1  [["id", 1]]
=> #<BigDecimal:7fe7a6bc7678,'0.6E2',9(27)>

我读到another post，可以使用ActiveRecord::Base.connection.execute运行原始sql命令。我应该得到类似“ 60.0”的东西，因为Item.first.style.retail_price.to_s #=> "60.0"相反，我得到了某种带有哈希值的数组：

[26] pry(main)> ActiveRecord::Base.connection.execute('SELECT  "items".* FROM "items"  ORDER BY "items"."id" ASC LIMIT 1; SELECT  "styles".* FROM "styles" WHERE "styles"."id" = ? LIMIT 1  [["id", 1]]')

   (0.2ms)  SELECT  "items".* FROM "items"  ORDER BY "items"."id" ASC LIMIT 1; SELECT  "styles".* FROM "styles" WHERE "styles"."id" = ? LIMIT 1  [["id", 1]]
=> [{"id"=>1, "size"=>"M", "color"=>"Purple", "status"=>"sellable", "price_sold"=>nil, "sold_at"=>nil, "style_id"=>1, "created_at"=>"2017-02-04 04:47:09.415828", "updated_at"=>"2017-02-04 04:47:09.415828", "clearance_batch_id"=>nil, 0=>1, 1=>"M", 2=>"Purple", 3=>"sellable", 4=>nil, 5=>nil, 6=>1, 7=>"2017-02-04 04:47:09.415828", 8=>"2017-02-04 04:47:09.415828", 9=>nil}]

另外，当我尝试相同的查询字符串是rails db时，我还会收到语法错误：

rails db
SQLite version 3.14.0 2016-07-26 15:17:14
Enter ".help" for usage hints.
sqlite> SELECT  "items".* FROM "items"  ORDER BY "items"."id" ASC LIMIT 1; SELECT  "styles".* FROM "styles" WHERE "styles"."id" = ? LIMIT 1  [["id", 1]];
1|M|Purple|sellable|||1|2017-02-04 04:47:09.415828|2017-02-04 04:47:09.415828|
Error: near "[["id", 1]": syntax error
sqlite>

3个问题：


ActiveRecord::Base.connection.execute需要什么字符串才能返回看起来像60.0的预期结果？
rails db需要什么字符串？
如何将rails console给我的sql字符串转换为可与ActiveRecord::Base.connection.execute和rails db一起使用的原始sql查询字符串的一般模式是什么？

Answer 1

如果只想从一个区域中选择一个字段，则必须在SQL的select子句中指定它。像这样

SELECT styles.retail_price FROM styles WHERE styles.id = 1

在这种情况下，如果使用rails的retail_price方法，则只能获取execute数组中的内容。如果只需要单个值，请使用select_value方法，而不要使用execute。喜欢

ActiveRecord::Base.connection.select_value("SELECT styles.retail_price FROM styles WHERE styles.id = 1")

由于此[["id", 1]]，在sqlite3终端中您将获得语法错误。这是用于准备好的语句的rails / applicative-level东西，但不是有效的SQL。您应该像我一样替换ID。