Arel和多态关联加入

Question

I have the following:

class BaseReseller < ActiveRecord::Base

  set_table_name "resellers"

  acts_as_nested_set

  has_many :merchants, :as => :merchant_owner, :dependent => :destroy
end

class IPSP < BaseReseller
end    

class Agent < BaseReseller
end

class Reseller < BaseReseller
end

class Merchant < ActiveRecord::Base
  belongs_to :merchant_owner, :polymorphic => true
end

class MerchantsController < ApplicationController
  ... 
  def index
    ...
    @merchants = Merchant.joins(:merchant_owner) # breaks!
  end
end

Notice how I am trying to join the Merchant with the polymorphic merchant_owner and get this: ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :merchant_owner .

@merchants = Merchant.includes(:merchant_owner) works initially, but when I start iterating over the @merchants array in the views, it breaks with the same error - seems to be so because we work with lazily loaded relations and only when a non-Arel method is invoked, it actually goes to the DB.

Any ideas? Does Arel support polymorphic association joins? Any workarounds? I can drop down to pure SQL but this is piggy.

Thanks

Answer 1

This may be too much to ask of Arel, I'm not sure of a great solution. In your case it looks like all of the merchant_owners share the same reseller table, so it makes some sense to want to join from the polymorphic association, but in general the polymorphic associations can relate to various models (for different tables) throughout your data model. This would get pretty ugly in SQL and probably pretty slow in the general case which is why I expect EagerLoadPolymorphicError exists. For your situation, it could make sense to work around with something like:

Merchant.joins("INNER JOIN resellers on merchant_owner_id = resellers.id")

of course it would be cleanest for this join to be a scope on your model so you could just do Merchant.joined_with_owners and the SQL relegated to the model layer. This wouldn't however work if other, non reseller models have many merchants.

I'm a bit surprised that you're seeing the same error with Merchant.includes(:merchant_owner), I don't see that with similar data even as I access various attributes but could be due to various differences.

Are trying to eager load for performance reasons or because you have additional Arel where clauses that depend on reseller attributes? If it's for performance, you may be best falling back on SQL...

Answer 2

Problem was a missing column in one of the tables. Rails does support polymorphic associations between STI hierarchies extremely well, but be careful regarding the correct columns for polymorphism, STI, etc.