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copied!<p>It looks like you have not specified <code>opinionable_type:string</code> column for your Opinion model.</p> <p>Try to update your migration in this manner:</p> <pre><code>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 </code></pre> <p>This will solve your second question and <code>Opinion.preload(:opinionable).all</code> should work well.</p> <p>You cann't do joins on polymorphic association because they can be located in different tables, which are detected after <code>Opinion</code> model is loaded. That why model needs column <code>opinionable_type</code>.</p> <p>If you try to do this you'll get next exception</p> <blockquote> <p><code>ActiveRecord::EagerLoadPolymorphicError</code>: Can not eagerly load the polymorphic association <code>:opinionable</code></p> </blockquote> <p>UPD: Added magic join ^_^</p> <pre><code>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 </code></pre> <p>Example:</p> <pre><code>Opinion.by_type(Answer).to_sql => "SELECT \"opinions\".* FROM \"opinions\" JOIN answers ON answers.id = opinions.opinionable_id AND opinions.opinionable_type = 'Answer'" </code></pre>

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