我知道，當你使用includes並指定在加入表where條款，你應該使用.references比較.references要求包括對eager_load
例如：
# will error out or throw deprecation warning in logs
users = User.includes(:orders).where("Orders.cost < ?", 20)
在軌道4,5或以後，您將收到如下錯誤：
Mysql2::Error: Unknown column 'Orders.cost' in 'where clause': SELECT customers.* FROM customers WHERE (Orders.cost < 100)
或者您將收到棄用警告：
DEPRECATION WARNING: It looks like you are eager loading table(s) (one of: users, addresses) that are referenced in a string SQL snippet. For example:
Post.includes(:comments).where("comments.title = 'foo'") Currently, Active Record recognizes the table in the string, and knows to JOIN the comments table to the query, rather than loading comments in a separate query. However, doing this without writing a full-blown SQL parser is inherently flawed. Since we don't want to write an SQL parser, we are removing this functionality. From now on, you must explicitly tell Active Record when you are referencing a table from a string:
Post.includes(:comments).where("comments.title = 'foo'").references(:comments)
If you don't rely on implicit join references you can disable the feature entirely by setting config.active_record.disable_implicit_join_references = true. (
SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "users"."email" AS t0_r2, "users"."created_at" AS t0_r3, "users"."updated_at" AS t0_r4, "addresses"."id" AS t1_r0, "addresses"."user_id" AS t1_r1, "addresses"."country" AS t1_r2, "addresses"."street" AS t1_r3, "addresses"."postal_code" AS t1_r4, "addresses"."city" AS t1_r5, "addresses"."created_at" AS t1_r6, "addresses"."updated_at" AS t1_r7 FROM "users" LEFT OUTER JOIN "addresses" ON "addresses"."user_id" = "users"."id" WHERE (addresses.country = 'Poland')
所以我們這樣做：
# added .references(:orders)
users = User.includes(:orders).where("Orders.cost < ?", 20).references(:orders)
它執行得很好：
SELECT "users"."id" AS t0_r0,
"users"."name" AS t0_r1,
"users"."created_at" AS t0_r2,
"users"."updated_at" AS t0_r3,
"orders"."id" AS t1_r0,
"orders"."cost" AS t1_r1,
"orders"."user_id" AS t1_r2,
"orders"."created_at" AS t1_r3,
"orders"."updated_at" AS t1_r4
FROM "users"
LEFT OUTER JOIN "orders"
ON "orders"."user_id" = "users"."id"
WHERE (orders.cost < 20)
我知道.includes僅僅是兩個方法的包裝：eager_load和preload。我知道，因爲我的查詢以上（在這個例子中orders）做一個連接表的過濾器，includes很聰明，知道要挑eager_load實現超過preload因爲preload不能處理這樣的查詢，因爲preload連接不表。
這裏是我困惑的地方。好的：所以在上面的那個查詢中：引擎蓋includes將使用eager_load實現。但請注意，當我爲相同的查詢明確使用eager_load（這是includes實質上是這樣做的）：我不需要使用.references！它運行查詢並加載數據就好了。沒有錯誤，沒有折舊警告：
# did not specify .references(:orders), and yet no error and no deprecation warning
users = User.eager_load(:orders).where("Orders.cost < ?", 20)
而且它沒有問題，執行完全相同的過程：
SELECT "users"."id" AS t0_r0,
"users"."name" AS t0_r1,
"users"."created_at" AS t0_r2,
"users"."updated_at" AS t0_r3,
"orders"."id" AS t1_r0,
"orders"."cost" AS t1_r1,
"orders"."user_id" AS t1_r2,
"orders"."created_at" AS t1_r3,
"orders"."updated_at" AS t1_r4
FROM "users"
LEFT OUTER JOIN "orders"
ON "orders"."user_id" = "users"."id"
WHERE (orders.cost < 20)
這似乎很奇怪。爲什麼需要爲includes版本的查詢指定.references，而不需要爲eager_load版本的查詢指定.references？我在這裏錯過了什麼？