The new TMDL model definition language

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Power BI Desktop updates for January 2025 Part three of a four-part series of blogs
This month includes some pretty minor updates (such as new ways to do treemaps), but we're breaking with our usual rules to highlight two features in preview: a new way to save files and a new language called TMDL for updating models. Introducing the Power BI Desktop updates for January 2025 The new Power BI Desktop file picker is very welcome The new TMDL model definition language (this blog) New ways of viewing treemaps, and of spacing them out We've been creating our idiosyncratic monthly blogs on Power BI updates since November 2016, and also deliver online and classroom Power BI courses.

Power BI Desktop updates for January 2025

Part three of a four-part series of blogs

This month includes some pretty minor updates (such as new ways to do treemaps), but we're breaking with our usual rules to highlight two features in preview: a new way to save files and a new language called TMDL for updating models.

Introducing the Power BI Desktop updates for January 2025
The new Power BI Desktop file picker is very welcome
The new TMDL model definition language (this blog)
New ways of viewing treemaps, and of spacing them out

We've been creating our idiosyncratic monthly blogs on Power BI updates since November 2016, and also deliver online and classroom Power BI courses.

Posted by Andy Brown on 03 February 2025

In this blog

Starting with TMDL
Starting as TMDL script
Making and applying changes
Changes which aren't supported
Getting help on writing TMDL

You can now write TMDL (Tabular Model Definition Language) to make changes to your data model, which for experienced developers will be a big time-saver.

If you know how to create SQL in SQL Server Management Studio you have a good idea of what scriptng in TMDL is going to involve (although the language is completely different, the principle of writing instructions to make changes is the same).

Starting with TMDL

The first thing you need to do is to turn on this preview feature:

See the first part of this blog for more details on how to enable preview features like this.

You can then click on the new tab which appears:

This tab only appears if you've enabled TMDL in preview.

You then see some sample script:

Ignore this: the easiest thing to do is just to choose to make changes to a table to get started with TMDL.

Starting as TMDL script

There are two ways to make changes to a table. Either choose to script them:

Right-click and choose this menu option.

Or alternatively create a new script window:

Optionally, click on the + button to add a new window for typing script.

You can then drag the table you want to modify onto your new window:

The visual clues of what's about to happen when you drag a table are excellent!

Making and applying changes

You can now - carefully - make changes to your model

Here I've changed the name of a column, and am about to apply this change. Notice that you can't undo this.

This change is immediately reflected in my model:

So you can now make changes to your report's underlying data model in yet another way!

Changes which aren't supported

When you start using this feature you'll see this warning:

You can click on the button to see a summary of what you can and can't do using this new preview feature.

If youi're connecting to an Analysis Services tabular model you will probably know what you're doing anyway, which is just as well since many additional rules apply. Otherwise, there aren't many restrictions on what you can and can't change using the new TMDL tab.

Getting help on writing TMDL

Here are two ways to get help on how to create TMDL:

Source of help	Notes
Microsoft	You can see how to write TMDL at this short URL, which redirects to this much longer one.
Wise Owl Training	We are planning to create a new TMDL course aimed at people who would like to be able to create and maintain data models using script. If this sounds like something which would be of interest, please (without any obligation on your part whatsoever) let us know.

An obvious advantage of scripting data models is that a development team can manage the code files in Git, and also keep track of different versions there.

Parts of this blog
Introducing the Power BI Desktop updates for January 2025 The new Power BI Desktop file picker is very welcome The new TMDL model definition language (this blog) New ways of viewing treemaps, and of spacing them out

Some other pages relevant to the above blogs include:

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Starting with TMDL

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Changes which aren't supported

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