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POUpdate Mapping Table Attribute in Many to Many Association
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copied!<p>I have a many to many association setup in Rails with ActiveRecord. Let's say the tables are </p> <pre><code>+------------+ +----------------+ +----------+ | categories | -- has many -= | category_items | =- has many -- | products | +------------+ +----------------+ +----------+ </code></pre> <p>In the category_items table I have the standard set of id's plus an extra attribute called "type": </p> <pre><code>id:int category_id:int product_id:int category_type:string </code></pre> <p>Thankfully, Rails provides some great helpers for making assignment in the mapping table a breeze. For example:</p> <pre><code>p = Product.first # p.id => 1 c = Category.first # c.id => 1 # now to make the assignment p.categories << c # or p.categories.create(c) </code></pre> <p>That's all well and good, but let's say I want to automatically update the "type" field with the class name of the other table. So for the examples given above, my category_items row would look like this:</p> <pre><code>id = 1 (or some number) category_id = 1 product_id = 1 category_type = nil </code></pre> <p>But I would like category_type to equal "Product". Is there a way for me to build a callback or define something in the association that would automatically set the category_type field? I know in polymorphic associations, you can use something like :source or :as to do something like this, but polymorphic associations in many to many associations throws an error.</p> <p>Any ideas?</p> <p>Thanks!</p>

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