Eager Load Depending on Type of Association in Ruby on Rails

Question

I have a polymorphic association (belongs_to :resource, polymorphic: true) where resource can be a variety of different models. To simplify the question assume it can be either a Order or a Customer.

If it is a Order I'd like to preload the order, and preload the Address. If it is a customer I'd like to preload the Customer and preload the Location.

The code using these associations does something like:

<%- @issues.each do |issue| -%>
<%- case issue.resource -%>
<%- when Customer -%>
<%= issue.resource.name %> <%= issue.resource.location.name %>
<%- when Order -%>
<%= issue.resource.number %> <%= issue.resource.address.details %>
<%- end -%>

Currently my preload uses:

@issues.preload(:resource)

However I still see n-plus-one issues for loading the conditional associations:

SELECT "addresses".* WHERE "addresses"."order_id" = ...
SELECT "locations".* WHERE "locations"."customer_id" = ...
...

What's a good way to fix this? Is it possible to manually preload an association?

Answer 1

You can do that with the help of ActiveRecord::Associations::Preloader class. Here is the code:

@issues = Issue.all # Or whatever query
ActiveRecord::Associations::Preloader.new.preload(@issues.select { |i| i.resource_type == "Order" }, { resource: :address })
ActiveRecord::Associations::Preloader.new.preload(@issues.select { |i| i.resource_type == "Customer" }, { resource: :location })

You can use different approach when filtering the collection. For example, in my project I am using group_by

groups = sale_items.group_by(&:item_type)
groups.each do |type, items|
  conditions = case type
  when "Product" then :item
  when "Service" then { item: { service: [:group] } }
end

ActiveRecord::Associations::Preloader.new.preload(items, conditions)

You can easily wrap this code in some helper class and use it in different parts of your app.

Answer 2

This is now working in Rails v6.0.0.rc1: https://github.com/rails/rails/pull/32655

You can do .includes(resource: [:address, :location])