Rails 包含条件 [英] Rails includes with conditions

问题描述

Rails > 3.2 是否可以在includes 方法生成的join 语句中添加条件?

Is is possible in Rails > 3.2 to add conditions to the join statement generated by the includes method?

假设我有两个模型，Person 和 Note.每个人有许多笔记，每个笔记属于一个人.每个笔记都有一个属性important.

Let's say I have two models, Person and Note. Each person has many notes and each note belong to one person. Each note has an attribute important.

我想找到所有的人只预加载重要的笔记.在 SQL 中，这将是:

I want to find all the people preloading only the notes that are important. In SQL that will be:

SELECT *
FROM people
LEFT JOIN notes ON notes.person_id = people.id AND notes.important = 't'

在 Rails 中，唯一类似的方法是使用 includes(注意:joins 不会预加载注释)，如下所示:

In Rails, the only similar way to do that is using includes (note: joins won't preload notes) like this:

Person.includes(:notes).where(:important, true)

但是，这将生成以下返回不同结果集的 SQL 查询:

However, that will generate the following SQL query which returns a different result set:

SELECT *
FROM people
LEFT JOIN notes ON notes.person_id = people.id
WHERE notes.important = 't'

请注意，第一个结果集包含所有人，第二个结果集仅包含与重要笔记相关联的人.

Please, notice that the first resultset includes all the people and the second one only the people associated to important notes.

另请注意 :conditions 自 3.1 起已弃用.

Also notice that :conditions are deprecated since 3.1.

推荐答案

Rails 5+ 语法:

Rails 5+ syntax:

Person.includes(:notes).where(notes: {important: true})

嵌套:

Person.includes(notes: [:grades]).where(notes: {important: true, grades: {important: true})

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