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copied!<p>The design is strange a bit. If you need nesting like </p> <pre><code>ProgramCategory > ProgramSubcategory > Program </code></pre> <p>then you need</p> <pre><code>class Program < ActiveRecord::Base belongs_to :program_subcategory end class ProgramSubcategory < ActiveRecord::Base belongs_to :program_category has_many :programs end class ProgramCategory < ActiveRecord::Base has_many :programs, :through => :program_subcategories has_many :program_subcategories end </code></pre> <p>This way when you create a program you can assign a subcategory to it. And this subcategory is already assigned to category, so you can access it via <code>program.program_subcategory.program_category</code></p> <p>And you do not need <code>program_category_id</code> foreign key in <code>programs</code> because program is not connected to category <em>directly</em>, but via subcategory.</p> <hr> <p><strong>UPDATE</strong></p> <blockquote> <ol> <li>Each Program has ONE Category.</li> <li>Each Program has ONE Subcategory.</li> <li>Each Category has MANY Subcategories</li> <li>Each Category has MANY Programs.</li> <li>Each Subcategory has ONE Category</li> <li>Each Subcategory has MANY Programs.</li> </ol> </blockquote> <p>Then I believe that my answer is still valid. You see, my structure is the same as your description except <code>Each Program has ONE Category</code> (because rails has no <code>belongs_to through</code>). you <code>has one</code> is actually <code>belongs_to</code> (because it can belong to only one).</p> <p>But as soon as <code>Each Program has ONE Subcategory</code> and <code>Each Subcategory has ONE Category</code> program's subcategory's category will be the ONLY program's category. You can have <code>p.program_category</code> by defining a method on a Program class:</p> <pre><code>def program_category program_subcategory.program_category end </code></pre> <hr> <p>Now for the part of </p> <blockquote> <p>I should be able to retrieve and also EDIT these:</p> <p>p.program_category</p> </blockquote> <p>Imagine you have a Program in subcategory Comedy from category Movies.</p> <p>You say you want to be able to EDIT programs category directly (if I understood correctly), like this:</p> <pre><code>p.program_category = ProgramCategory.find_by_name("Sports") </code></pre> <p>But what you expect to be program's subcategory then? As soon as Sports have many subcategories? Do you expect it to be blank?</p> <p>So in this design the only way to change program's category is to change program's subcategory:</p> <pre><code>p.program_subcategory = ProgramSubcategory.find_by_name("Tennis") </code></pre> <p>And this will manke program's category == Sports, because Tennis belongs to Sports.</p> <p><strong>Note:</strong> If you really want sometimes to change program's category directly, leaving its subcategory blank it requires another design of course. I do not think it is very difficult but it requires more work with less help from Rails AR Associations magic.</p>

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