在Rails查询中获取关联模型的属性

Question

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

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.

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

But is it possible to do something like the following:

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

Answer 1

Try this:

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

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.

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')

Answer 2

Unfortunately, I don't think that we could do it through AR in a single query. You could do a nested query below to retrieve it in two queries to the database:

Step.includes(:projects)
    .where(projects: { id: Projects.includes(:collections)
    .where(collections: { id: @collections.id }).pluck(:id) } )
    .pluck(:id)