Jinja is a templating language widely used in many domains, including
web development and data engineering. At dbt Labs, we extensively use
Jinja to enable dynamic SQL rendering via the dbt fusion engine, a
popular data transformation tool. However the flexibility of Jinja
templates (i.e., lack of types) can lead to errors in the rendered SQL
code, which can be costly to debug and fix. To address this issue, we
present the design and implementation of TypeJinja: a typed version of
the Jinja language and its integration with the dbt fusion
engine. TypeJinja statically analyzes Jinja templates and their
context to detect type errors, before rendered SQL code ends up being
used, providing developers with early feedback.  Furthermore,
TypeJinja is designed to be extensible, allowing it to be adapted to
different use cases. We have evaluated TypeJinja on our codebase and
found 30 previously unknown type errors, demonstrating its
effectiveness in improving code quality and reducing debugging
effort. TypeJinja is now part of the publicly available GitHub
repository that hosts the dbt fusion engine.