Why does Rails orphan join records for polymorphic through associations?

polymorphic-associations, ruby-on-rails, ruby-on-rails-3

Solution

ActiveRecord can't know that proposal.fee_summaries is reverse association from fee_summary.proposal. It's because you can define your own association name, have additional constraints on it, etc. - to automatically derive which associations are reverse of which would be insanely hard, if not impossible. So even for most straightforward cases you need to tell it explicitly via `inverse_of` option on the association declaration. Here's an example for a simple direct association:

class Proposal < ActiveRecord::Base
  has_many :proposal_sections, :inverse_of => :proposal
end

class ProposalSection < ActiveRecord::Base
  belongs_to :proposal, :inverse_of => :proposal_sections
end

2.0.0-p353 :001 > proposal = Proposal.new
 => #<Proposal id: nil, created_at: nil, updated_at: nil> 
2.0.0-p353 :002 > section = proposal.proposal_sections.new
 => #<ProposalSection id: nil, proposal_id: nil, created_at: nil, updated_at: nil> 
2.0.0-p353 :003 > section.proposal
 => #<Proposal id: nil, created_at: nil, updated_at: nil>

Unfortunately, `inverse_of` does not support indirect (`through`) and polymorphic assocations. So in your case there's no easy way to make it work. The only workaround I see is to persist the records (use `create`), so AR can just query the relationships by key and return correct results.

Check the docs for more examples and explanations: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods

Problem

A proposal document can be split up into many different section types (text, fees, schedule) etc Here it's modelled using a polymoprhic association on the join table. ``` class Proposal < ActiveRecord::Base has_many :proposal_sections has_many :fee_summaries, :through => :proposal_sections, :source => :section, :source_type => 'FeeSummary' end class ProposalSection < ActiveRecord::Base belongs_to :proposal belongs_to :section, :polymorphic => true end class FeeSummary < ActiveRecord::Base has_many :proposal_sections, :as => :section has_many :proposals, :through => :proposal_sections end ``` Whilst #create works fine ``` summary = @proposal.fee_summaries.create summary.proposal == @propsal # true ``` #new doesnt ``` summary = @proposal.fee_summaries.new summary.proposal -> nil ``` Should it be returning nil? On a regular has_many and belongs_to initialized yet unpersisted records will still return their parent assocation (built in memory). Why won't this work and is it intended behaviour? Schema.rb ``` create_table "fee_summaries", :force => true do |t| t.datetime "created_at", :null => false t.datetime "updated_at", :null => false end create_table "proposal_sections", :force => true do |t| t.integer "section_id" t.string "section_type" t.integer "proposal_id" t.datetime "created_at", :null => false t.datetime "updated_at", :null => false end create_table "proposals", :force => true do |t| t.datetime "created_at", :null => false t.datetime "updated_at", :null => false end ``` ruby 2.0 rails 3.2.14

Original source