我知道，当你使用includes并指定在加入表where条款，你应该使用.references比较.references要求包括对eager_load
例如：
# will error out or throw deprecation warning in logs
users = User.includes(:orders).where("Orders.cost < ?", 20)
在轨道4,5或以后，您将收到如下错误：
Mysql2::Error: Unknown column 'Orders.cost' in 'where clause': SELECT customers.* FROM customers WHERE (Orders.cost < 100)
或者您将收到弃用警告：
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')
所以我们这样做：
# added .references(:orders)
users = User.includes(:orders).where("Orders.cost < ?", 20).references(:orders)
它执行得很好：
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仅仅是两个方法的包装：eager_load和preload。我知道，因为我的查询以上（在这个例子中orders）做一个连接表的过滤器，includes很聪明，知道要挑eager_load实现超过preload因为preload不能处理这样的查询，因为preload连接不表。
这里是我困惑的地方。好的：所以在上面的那个查询中：引擎盖includes将使用eager_load实现。但请注意，当我为相同的查询明确使用eager_load（这是includes实质上是这样做的）：我不需要使用.references！它运行查询并加载数据就好了。没有错误，没有折旧警告：
# did not specify .references(:orders), and yet no error and no deprecation warning
users = User.eager_load(:orders).where("Orders.cost < ?", 20)
而且它没有问题，执行完全相同的过程：
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)
这似乎很奇怪。为什么需要为includes版本的查询指定.references，而不需要为eager_load版本的查询指定.references？我在这里错过了什么？