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POElegantly selecting attributes from has_many :through join models in Rails
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  1. POElegantly selecting attributes from has_many :through join models in Rails
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    copied!<p>I'm wondering what the easiest/most elegant way of selecting attributes from join models in has_many :through associations is.</p> <p>Lets say we have Items, Catalogs, and CatalogItems with the following Item class:</p> <pre><code>class Item &lt; ActiveRecord::Base has_many :catalog_items has_many :catalogs, :through =&gt; :catalog_items end </code></pre> <p>Additionally, lets say that CatalogueItems has a position attribute and that there is only one CatalogueItem between any catalog and any item. </p> <p>The most obvious but slightly frustrating way to retrieve the position attribute is:</p> <pre><code>@item = Item.find(4) @catalog = @item.catalogs.first @cat_item = @item.catalog_items.first(:conditions =&gt; {:catalog_id =&gt; @catalog.id}) position = @cat_item.position </code></pre> <p>This is annoying because it seems that we should be able to do @item.catalogs.first.position since we have completely specified which position we want: the one that corresponds to the first of @item's catalogs. </p> <p>The only way I've found to get this is:</p> <pre><code>class Item &lt; ActiveRecord::Base has_many :catalog_items has_many :catalogs, :through =&gt; :catalog_items, :select =&gt; "catalogue_items.position, catalogs.*" end </code></pre> <p>Now I can do Item.catalogs.first.position. However, this seems like a bit of a hack - I'm adding an extra attribute onto a Catalog instance. It also opens up the possibility of trying to use a view in two different situations where I populate @catalogs with a Catalog.find or with a @item.catalogs. In one case, the position will be there, and in the other, it won't. </p> <p>Does anyone have a good solution to this?</p> <p>Thanks.</p>
 

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