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copied!<p>In this case, I would lean against making a separate <code>Spend</code> model, since all it does is calculate data that is already present in the database. Unless you have severe caching requirements (and I doubt it in your case), you can use simple instance methods to retrieve the data you want.</p> <p>First figure out how your Usage and Price models are related. Since you seem to be associating them by id, it appears to be a one-to-one relationship (correct me if I'm wrong on this). However, associating by assuming they have the same primary key is a dangerous approach - rather have one model point to the other using a foreign key. We'll pick the <code>Price</code> model to hold a primary key for <code>Usage</code>, but the reverse can also work. You'll need to add a column using a migration like this:</p> <pre><code>def change add_column :prices, :usage_id, :integer end </code></pre> <p>Your models should then look like this:</p> <pre><code>class Usage < ActiveRecord::Base has_one :price def spend usage * price.price end end class Price < ActiveRecord::Base belongs_to :usage end </code></pre> <p>And you can find your <code>spend</code> value for an individual <code>usage</code> item like this:</p> <pre><code>usage = Usage.find(some_id) puts usage.spend </code></pre> <p>Or you can get multiple 'spends' like this:</p> <pre><code>Usage.include(:price).each do |usage| puts usage.spend end </code></pre> <p>I've left out any reference to month, as I'm not sure how you are using it or if it's needed at all for calculating <code>spend</code>.</p>

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