Question

my problem is following. How can I joins belongs_to association from polymorphic model

There is situation

opinion.rb

class Opinion < ActiveRecord::Base
    belongs_to :opinionable, :polymorphic => true
    belongs_to :category
end

answer.rb

class Answer < ActiveRecord::Base
    has_many :opinions, :as => :opinionable
end

How can i do following

Opinion.joins(:opinionabe).all

it will throw

ArgumentError: You can’t create a polymorphic belongs_to join without specifying the polymorphic class!

How can i specific which class i want to join?

Second question. How to preload it?

Opinion.preload(:opinionable).all

works fine. It will do query for each class in belongs_to.

But. if i want to do something like

Opinion.preload(:opinionable => :answer_form).all

there is problem because one model has this association and second hasn’t. So it will throw exception.

So how i can do something like

Opinion.preload(:answer => :answer_form, :another_belongs_to_model).all

?

Thanks, David!

Answer 1

It looks like you have not specified opinionable_type:string column for your Opinion model.

Try to update your migration in this manner:

class CreateOpinions < ActiveRecord::Migration
  def self.up
    create_table :opinions do |t|
      t.integer :opinionable_id
      t.string  :opinionable_type

      # ... other fields

      t.timestamps
    end
  end

  def self.down
    drop_table :opinions
  end
end

This will solve your second question and Opinion.preload(:opinionable).all should work well.

You cann’t do joins on polymorphic association because they can be located in different tables, which are detected after Opinion model is loaded. That why model needs column opinionable_type.

If you try to do this you’ll get next exception

ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :opinionable

UPD: Added magic join ^_^

class Opinion < ActiveRecord::Base
  belongs_to :opinionable, :polymorphic => true

  belongs_to :opinionable_answer, :foreign_key => :opinionable_id, :class_name => "Answer"

  scope :by_type, lambda { |type| joins("JOIN #{type.table_name} ON #{type.table_name}.id = #{Opinion.table_name}.opinionable_id AND #{Opinion.table_name}.opinionable_type = '#{type.to_s}'") }
end

Example:

Opinion.by_type(Answer).to_sql
  => "SELECT \"opinions\".* FROM \"opinions\" JOIN answers ON answers.id = opinions.opinionable_id AND opinions.opinionable_type = 'Answer'"