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POCan't get an STI/ActiveRecord model to do what I expect
 

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  1. POCan't get an STI/ActiveRecord model to do what I expect
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    <p>I'm trying to build a simple survey/questionnaire app. Surveys have <code>Questions</code>; most questions consist of a single content field (the question itself), for which the survey taker will write in a free-text response. (There are also a couple of other fields not relevant to this discussion.) However, users can also create <code>MultipleChoiceQuestions</code> or <code>LikertQuestions</code> (e.g., answers on a 1 - 5 scale). (In the case of <code>MultipleChoiceQuestions</code>, there will be another model called <code>Answer</code> such that a <code>MultipleChoiceQuestion</code> has_many <code>Answers</code>). Here are my design choices, so far as I know:</p> <p>1) Inherit from Question:</p> <pre><code>class Question &lt; ActiveRecord::Base attr_accessible :id, :content end class MultipleChoiceQuestion &lt; Question attr_accessible :type end class LikertQuestion &lt; Question attr_accessible :type, :min, :max, :label_min, label_max end </code></pre> <p>2) Use a module/mixin with the shared attributes and methods:</p> <pre><code>module Question @content, @id def method1 end end class MultipleChoiceQuestion &lt; ActiveRecord::Base include Question end class LikertQuestion &lt; ActiveRecord::Base include Question attr_accessible :type, :min, :max, :label_min, label_max end </code></pre> <p>This seems a clear-cut case of inheritance, so I went with option 1. Since then, I can't get it to work. Single Table Inheritance seemed simple enough, so I gave <code>MultipleChoiceQuestion</code> and <code>LikertQuestion</code> each <code>type:string</code> in their schema. Here are the schema for each (from db/schema.rb):</p> <pre><code> create_table "questions", :force =&gt; true do |t| t.integer "parent" t.string "type" t.string "content" t.datetime "created_at", :null =&gt; false t.datetime "updated_at", :null =&gt; false t.integer "survey_id" end create_table "multiple_choice_questions", :force =&gt; true do |t| t.datetime "created_at", :null =&gt; false t.datetime "updated_at", :null =&gt; false t.string "type" end create_table "likert_questions", :force =&gt; true do |t| t.integer "min" t.integer "max" t.string "label_min" t.string "label_max" t.datetime "created_at", :null =&gt; false t.datetime "updated_at", :null =&gt; false t.string "type" end </code></pre> <p>If I implement option 1, above, then MultipleChoiceQuestion and LikertQuestion somehow do not actually include any of their unique fields as specified in schema.rb; instead, they have only the inherited fields from Question. See console output:</p> <pre><code>1.9.3p392 :001 &gt; Question =&gt; Question(id: integer, parent: integer, content: string, created_at: datetime, updated_at: datetime, survey_id: integer) 1.9.3p392 :002 &gt; LikertQuestion =&gt; LikertQuestion(id: integer, parent: integer, content: string, created_at: datetime, updated_at: datetime, survey_id: integer) 1.9.3p392 :003 &gt; MultipleChoiceQuestion =&gt; MultipleChoiceQuestion(id: integer, parent: integer, content: string, created_at: datetime, updated_at: datetime, survey_id: integer) 1.9.3p392 :004 &gt; LikertQuestion.new(:min =&gt; 3) ActiveRecord::UnknownAttributeError: unknown attribute: min </code></pre> <p>Somebody on StackOverflow said that Question should be an abstract class. But if I add <code>self.abstract_class = true</code> to Question.rb, then I get the following:</p> <pre><code>1.9.3p392 :001 &gt; Question =&gt; Question(abstract) 1.9.3p392 :002 &gt; LikertQuestion =&gt; LikertQuestion(id: integer, min: integer, max: integer, label_min: string, label_mid: string, label_max: string, created_at: datetime, updated_at: datetime, type: string) 1.9.3p392 :003 &gt; MultipleChoiceQuestion =&gt; MultipleChoiceQuestion(id: integer, created_at: datetime, updated_at: datetime, type: string) 1.9.3p392 :004 &gt; LikertQuestion.new(:content =&gt; "foo") ActiveRecord::UnknownAttributeError: unknown attribute: content </code></pre> <p><code>LikertQuestion</code> and <code>MultipleChoiceQuestion</code> show <em>only</em> their unique fields and do not inherit fields from the parent.</p> <p>1) What am I missing here? Regardless of whether inheritance is the optimal solution, I must be overlooking something obvious.</p> <p>2) Should I be using the module approach instead of inheritance? As I mentioned, inheritance seemed like a no-brainer: <code>LikertQuestion</code> and <code>MultipleChoiceQuestion</code> really are kinds of <code>Questions</code>. If I use the module approach, I lose the ability to say things like <code>survey.questions()</code>, <code>survey.questions.build()</code>, and presumably other handy stuff. What do Rails hotshots do in this situation? I'll do whatever that is.</p> <p>No posts on StackOverflow offer a very comprehensive discussion of the pros and cons of subclassing vs. mixin.</p> <p>Using Ruby 1.9.3 (though thinking of switching to 2.0), Rails 3.2.3.</p>
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    1. CORails 1.9.3 or Ruby 1.9.3?
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    2. CO@sosborn: I think it is pretty obvious that "Rails 1.9.3" is just a typo and they're using *Ruby* 1.9.3.
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    3. CO@mu, I know it is a mistake but I don't think it was a typo.
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