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POSelecting Rails Models via Virtual Attributes
 

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    <p>I have the two rails models <code>Section</code> &amp; <code>SectionRevision</code>. A Section is mostly just a container that holds all the Revisions relating to itself. So most of the attributes for the <code>Section</code> are basically stored within the <code>SectionRevision</code> model so there's a history of Revisions that can be reverted back to at any time.</p> <p>Sometimes I need to access the attributes for the latest revision from the Sections Model so I've created some Virtual Attributes to account for this.</p> <hr> <p>Each model has the attributes as defined in these migrations:</p> <p>Section:</p> <pre><code>class CreateSections &lt; ActiveRecord::Migration def change create_table :sections do |t| t.integer "page_id", :null =&gt; false t.timestamps t.datetime "deleted_at" end add_index("sections", "page_id") add_index("sections", "current_revision_id") end end </code></pre> <p>SectionRevision:</p> <pre><code>class CreateSectionRevisions &lt; ActiveRecord::Migration def change create_table :section_revisions do |t| t.integer "section_id", :null =&gt; false t.integer "parent_section_id" t.integer "position" t.string "title", :default =&gt; "", :null =&gt; false t.text "body", :null =&gt; false t.timestamps end add_index("section_revisions", "section_id") add_index("section_revisions", "parent_section_id") end end </code></pre> <hr> <p>And the models:</p> <p>SectionRevision:</p> <pre><code>class SectionRevision &lt; ActiveRecord::Base belongs_to :section, :class_name =&gt; 'Section', :foreign_key =&gt; 'section_id' belongs_to :parent_section, :class_name =&gt; 'Section', :foreign_key =&gt; 'parent_section_id' def parsed_json return JSON.parse(self.body) end end </code></pre> <p>Section:</p> <pre><code>class Section &lt; ActiveRecord::Base belongs_to :page has_many :revisions, :class_name =&gt; 'SectionRevision', :foreign_key =&gt; 'section_id' has_many :references def current_revision self.revisions.order('created_at DESC').first end def position self.current_revision.position end def parent_section self.current_revision.parent_section end def children Sections.where(:parent_section =&gt; self.id) end end </code></pre> <hr> <p>As you can see <code>Section</code> has a couple of virtual attributes like, <code>parent_section</code>,<code>current_revision</code> &amp; <code>position</code>. </p> <p>The problem being now I would like to create a virtual attribute, <code>children</code> that selects all sections where the virtual attribute <code>parent_section.id</code> is equal to <code>self.id</code>. Is this possible at all? I know the above code won't work as its doing a query for a column that doesn't exist - and I'm not sure how to access the Model instances from within the model 'Sections' doesn't appear to work. </p> <p>Can a perform a selection based on virtual attributes?</p> <hr> <p>I've updated the model based on ProGNOMmers answer and get the following:</p> <pre><code>class Section &lt; ActiveRecord::Base has_many :revisions, :class_name =&gt; 'SectionRevision', :foreign_key =&gt; 'section_id' #Need to somehow modify :child_revisions to only be selected if it is the section_id's current_revision? has_many :child_revisions, :class_name =&gt; 'SectionRevision', :foreign_key =&gt; 'parent_section_id' has_many :children, :through =&gt; :child_revisions, :source =&gt; :section end </code></pre> <p>Circumstance 1: This works perfectly fine.</p> <pre><code>1.9.3p392 :040 &gt; section =&gt; #&lt;Section id: 3, page_id: 10, created_at: "2013-04-02 01:31:42", updated_at: "2013-04-02 01:31:42", deleted_at: nil&gt; 1.9.3p392 :041 &gt; sub_section =&gt; #&lt;Section id: 4, page_id: 10, created_at: "2013-04-04 10:19:33", updated_at: "2013-04-04 10:19:33", deleted_at: nil&gt; 1.9.3p392 :042 &gt; revision1 =&gt; #&lt;SectionRevision id: 5, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 10:21:46", updated_at: "2013-04-04 21:55:10", position: 3, parent_section_id: nil&gt; 1.9.3p392 :043 &gt; revision2 =&gt; #&lt;SectionRevision id: 6, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 12:29:19", updated_at: "2013-04-04 21:55:15", position: 3, parent_section_id: 3&gt; 1.9.3p392 :044 &gt; sub_section.current_revision SectionRevision Load (0.6ms) SELECT `section_revisions`.* FROM `section_revisions` WHERE `section_revisions`.`section_id` = 4 ORDER BY created_at DESC LIMIT 1 =&gt; #&lt;SectionRevision id: 6, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 12:29:19", updated_at: "2013-04-04 21:55:15", position: 3, parent_section_id: 3&gt; 1.9.3p392 :045 &gt; section.children =&gt; [#&lt;Section id: 4, page_id: 10, created_at: "2013-04-04 10:19:33", updated_at: "2013-04-04 10:19:33", deleted_at: nil&gt;] </code></pre> <p>Circumstance 2:</p> <pre><code>1.9.3p392 :021 &gt; section =&gt; #&lt;Section id: 3, page_id: 10, created_at: "2013-04-02 01:31:42", updated_at: "2013-04-02 01:31:42", deleted_at: nil&gt; 1.9.3p392 :022 &gt; sub_section =&gt; #&lt;Section id: 4, page_id: 10, created_at: "2013-04-04 10:19:33", updated_at: "2013-04-04 10:19:33", deleted_at: nil&gt; 1.9.3p392 :023 &gt; revision1 =&gt; #&lt;SectionRevision id: 5, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 10:21:46", updated_at: "2013-04-04 10:24:22", position: 3, parent_section_id: 3&gt; 1.9.3p392 :024 &gt; revision2 =&gt; #&lt;SectionRevision id: 6, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 12:29:19", updated_at: "2013-04-04 12:29:19", position: 3, parent_section_id: nil&gt; 1.9.3p392 :025 &gt; sub_section.current_revision SectionRevision Load (0.7ms) SELECT `section_revisions`.* FROM `section_revisions` WHERE `section_revisions`.`section_id` = 4 ORDER BY created_at DESC LIMIT 1 =&gt; #&lt;SectionRevision id: 6, section_id: 4, title: "test", body: "[{\"type\":\"testbody\"}]", created_at: "2013-04-04 12:29:19", updated_at: "2013-04-04 12:29:19", position: 3, parent_section_id: nil&gt; 1.9.3p392 :026 &gt; section.children Section Load (0.6ms) SELECT `sections`.* FROM `sections` INNER JOIN `section_revisions` ON `sections`.`id` = `section_revisions`.`section_id` WHERE `section_revisions`.`parent_section_id` = 3 =&gt; [#&lt;Section id: 4, page_id: 10, created_at: "2013-04-04 10:19:33", updated_at: "2013-04-04 10:19:33", deleted_at: nil&gt;] </code></pre> <p>In circumstance 2 I would like <code>section.children</code> to return <code>=&gt; []</code> as <code>sub_section.current_revision.parent_section_id = nil</code> and not <code>section.id</code>.</p> <p>In other words <code>section.children</code> should return all <code>Sections</code> where <code>.current_revision.parent_section_id = section.id</code> but I can't query that as <code>.current_revision</code> is a virtual attribute.</p> <p>Is it possible to maybe turn <code>Section.current_revision</code> in to some sort of association? Or maybe the only way is to add a <code>current_revision</code> column to the <code>sections table</code>?</p>
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