查看关联记录中存在的所有记录

Question

Goal: I want to show all Category that are currently in existence in the Listing model.

For instance, a Listing can have many categories. Say I have 100 categories and 10 listings but only 20 of those categories are currently being used in the Listing.category_ids, I only want those to appear.

I have tried:

@categories = Category.all.where(id: @listings.each {|listing| listing.category_ids})

@categories = Category.where(id: Listing.all.includes(:category_ids)).order(name: :asc)

@categories = Category.where(id: [@listings.each {|listing| listing.category_ids}])

And some other similar ways. BTW, i use Postgres.

Both of these attempts have only appeared the first category within the Category model, even though that category isn't being used.

Models:

Listing

has_and_belongs_to_many :categories

Categories:

has_and_belongs_to_many :listings

I then have a Categories_Listings join table too t for it all. That is where the associations are stored.

Answer 1

You just need a join for that. Try

Category.joins(:listings)

In this way you should see just rows which have a match in listings