What is rabl?

RABL, short for Ruby API Builder Language, is a Ruby library that gives developers a clean, template-based way to define what data their web application sends back as JSON, XML, or a few other formats. It is designed for Rails and similar Ruby frameworks where an application needs to expose data through an API, meaning it answers requests from other software rather than serving a traditional web page. The problem it addresses is that Ruby on Rails comes with a built-in shortcut to convert database records to JSON, but that shortcut is limited. It can only include fields exactly as they are stored, cannot easily rename things, and becomes messy when you need to combine data from multiple sources or include conditional information. RABL solves this by letting you write a small view file that describes the desired output shape, much like you would write an HTML template for a regular web page. In a RABL template you can pick which fields to include, rename them, add calculated values, nest related records as children, pull in pieces from other templates (called partials), and show or hide nodes based on conditions. The template approach keeps this logic inside the view layer of the application rather than scattering it across database models or controllers. Installation is straightforward: add the gem to your project's Gemfile and run the bundler command. It works with Rails, Padrino, and Sinatra with minimal configuration. A global configuration block lets you adjust caching behavior, control whether output includes wrapper keys around each object, set the JSON parser, and toggle a few other options, but none of that is required to get started. Rabl has been maintained since around 2011 and has a detailed wiki covering setup for different frameworks, caching strategies, and common patterns for complex API responses.