An Introduction to the Tabular Model Definition Language (TMDL) in Power BI

This video introduces the Tabular Model Definition Language (TMDL) as a powerful new feature for Power BI developers. The central theme is that TMDL bridges the gap between semantic modeling and a true developer workflow, making data models easier to manage, version control, and share. It’s presented as a “new best friend” for anyone looking to enhance their Power BI development process.

Enhanced Developer Efficiency and Safety

The TMDL view provides a rich code editor directly within Power BI, complete with features like Find and Replace, IntelliSense, and keyboard shortcuts. The video demonstrates this by performing a bulk rename of tables (removing “dim_” and “fact_” prefixes) in a single operation. A key takeaway is the ability to create a backup script of the model. When the bulk change breaks the report, the presenter easily reverts to the original state by applying the backup, highlighting TMDL’s role in providing a safety net for developers.

Increased Reusability and Collaboration

TMDL significantly improves the reusability of model components. The presenter shows how to copy the TMDL script for a complete Calendar table and paste it into a new Power BI file. Unlike just sharing DAX, this method transfers the entire table definition, including columns, rows, and all its properties (like ‘Sort by column’ settings). This makes sharing and standardizing model components across projects seamless.

Advanced Control and Transparency

A major conclusion is that TMDL unlocks advanced modeling capabilities that were previously only available through third-party tools like Tabular Editor. The video demonstrates creating a “Perspective” directly within the TMDL view. In an even more powerful example, the presenter uses a TMDL script to change a table’s storage mode from ‘Import’ back to ‘DirectQuery’—an action the Power BI user interface explicitly prevents. This showcases TMDL’s ability to provide granular control and override UI limitations, offering more transparency into the semantic model’s structure.

Mentoring question

What is one repetitive or difficult task in your current Power BI development process that could be simplified or automated using TMDL’s script-based approach?

Source: https://youtube.com/watch?v=WGMIoteHMs4&si=qV49Ruo_dARfDzca