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    <p>Okay, thanks to <a href="https://stackoverflow.com/users/87189/tadman">tadman</a> for pushing me in the right direction.</p> <p>I digged somewhat deeper (especially in the log files) and what i found is a little bit weird.</p> <p>The problem was caused by the number of selected columns. If one selects only one column and counts the result</p> <pre><code>my_meal.noodles.select("distinct color").count </code></pre> <p>ActiveRecord creates the following SQL statement:</p> <pre><code>SELECT COUNT(distinct color) AS count_id FROM "NOODLES" WHERE ("NOODLES".meal_id = 295) </code></pre> <p>In case one selects two or more columns and applies <code>count</code> to it</p> <pre><code>my_meal.noodles.select("distinct color, shape").count </code></pre> <p>ActiveRecord forgets about that select clause and creates:</p> <pre><code>SELECT COUNT(*) AS count_id FROM "NOODLES" WHERE ("NOODLES".meal_id = 295) </code></pre> <p>This may be right, since (SQL's) <code>COUNT</code> allows only one or less columns as parameters. Add a <code>group</code> before the <code>count</code> and anything is fine:</p> <pre><code>my_meal.noodles.select("distinct color, shape").group("color, shape").count SELECT COUNT(*) AS count_all, color, shape AS color_shape FROM "NOODLES" WHERE ("NOODLES".meal_id = 295) GROUP BY color, shape </code></pre> <p>Apart from this <code>AS color_shape</code> it is exact what i expected. BUT... only it returns this:</p> <pre><code>&gt;&gt; my_meal.noodles.select("distinct color, shape").group("color, shape").count =&gt; {star=&gt;309, circle=&gt;111, spaghetti=&gt;189, square=&gt;194, triangle=&gt;179, bowtie=&gt;301, shell=&gt;93, letter=&gt;230} &gt;&gt; my_meal.noodles.select("distinct color, shape").group("color, shape").count.class =&gt; ActiveSupport::OrderedHash </code></pre> <p>This weird return value is (apart from order which depends on the DB) identical with the result and return value of</p> <pre><code>my_meal.noodles.group("shape").count </code></pre> <p>Conclusion:<br> As <a href="http://metautonomo.us/2010/05/11/activerecord-relation-vs-arel/" rel="nofollow noreferrer">pointed out here</a> there is still a gap between relations (may they be mathematical or arel relations) and ActiveRecord::Relations.<br> I can see the advantages of pressing the result in the patterns of a model as often as possible (at least in the context of a Rails app).<br> However real relations are not the result of the <em>combination</em> of several operations but the result of the <em>concatenation</em> of those operations. In general the chainability of ActiveRecord::Relations is a great thing but there are some design decisions i cannot follow.<br> If you can't depend on the assurance that each action returns a new relation to work with, it looses much of its inuitional appeal.</p> <p>As for the solution of my problem, i will use the above-mentioned <code>group</code> solution and some kind of dirty workaround for the count operation:</p> <pre><code>my_meal.noodles.select("distinct color, shape").group("color, shape").all.count </code></pre> <p>This compresses the results to an acceptable minimum before pulling them out of the database and creating expensive objects just to count them. Alternatively one could use a handwritten SQL query, but why have a Rails and do not use it, huh? ;-)</p> <p>Thanks for your help,<br> Tim</p>
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