Pluck公司相关模型的Rails中查询属性 [英] Pluck associated model's attribute in Rails query

问题描述

在我的Rails应用程序，收藏有很多的项目，和项目有很多的步骤

In my rails app, collections have many projects, and projects have many steps.

我想抓住的一个集合的项目步骤，所有的ID，我想知道如果我能做到这一切在一个查询。

I'd like to grab all the ids of steps in a collection's projects, and I'm wondering if I can do it all in one query.

例如，我知道我可以做以下

For example, I know I can do the following

step_ids = []
@collection.projects.each do |project|
    project.steps.each do |step|
       step_ids << step.id
    end
end

但它是可以做到类似如下：

But is it possible to do something like the following:

@ collection.projects.include（：步）.pluck（step.id） //这里的语法不正确

@collection.projects.include(:steps).pluck("step.id") // syntax here is not correct

推荐答案

试试这个：

Step.joins(:project).where(projects: { collection_id: @collection.id }).pluck(:'steps.id')

请注意使用项目的连接，然后项目的where子句。第一对应belongs_to的关系，后者是db表的名称。

Note the use of project for the joins, and then projects for the where clause. The first corresponds to the belongs_to relationship, and the latter is the name of the db table.

编辑：在项目和收藏，并假设一个项目belongs_to的一个project_collection（然后的has_many之间的许多一对多的关系的情况：通过收藏，：project_collection ）

edit: in the case of a many-to-many relationship between projects and collections, and assuming a project belongs_to a project_collection (and then has_many :collections, through :project_collection)

Step.joins(:project => :project_collection)
    .where(project_collections: { collection_id: @collection.id })
    .pluck(:'steps.id')

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