Checking a single attribute on a model for validity doesn’t seem to be possible in Rails. Or at least I couldn’t find a quick answer googling around or looking through the ActiveRecord code.
What I really want to do is pass a hash into valid? with the attributes I want to validate or have an attribute_valid? method. This level of granularity is often useful in AJAX heavy apps. For instance, checking for username availability and validity as the user types during username selection.
Here’s a quick solution for a User model.
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