areL

Arel is a Ruby library that provides a flexible, database-agnostic way to construct SQL queries. It represents queries as an abstract syntax tree and can be used independently of a specific object-relational mapper.

Core components include tables, nodes, and managers. Arel::Table models a database table; Arel::Nodes provide a range of elements such as columns and operators; Arel::SelectManager builds SELECT statements and supports joins and grouping. Arel employs a visitor pattern, with visitors like Arel::Visitors::ToSql translating the AST into SQL that targets a database dialect.

Arel originated within the Ruby on Rails ecosystem and is commonly used by ActiveRecord to generate SQL. While ActiveRecord offers higher-level query APIs, Arel underpins more complex query construction and can be used directly for advanced cases.

It is distributed as a gem and can be used outside Rails for building queries programmatically. It offers database portability and composability but has a learning curve and is more low-level than typical ActiveRecord use; for simple queries, raw SQL or higher-level ActiveRecord methods may be preferable. See also: ActiveRecord, Ruby on Rails, SQL.