TMDL code Skill

Tabular Model Definition Language (TMDL) is a declarative language that represents a Power BI semantic model in text. This skill provides important context when interacting directly with TMDL code.

TMDL Semantics / General Rules

A TMDL object is declared by specifying the TOM object type followed by its name
TMDL definition uses a folder structure, where some objects such as tables, culture, perspectives, roles are defined in separate files.
Objects like partition or measure have default properties that can be assigned after the equals (=) sign that specify the PowerQuery expression or DAX expression respectively.

DO:

Always learn from existing examples and patterns in the code. For example, existing naming conventions.
Object names must be enclosed in single quotes if they contain spaces or special characters such as .,=,:,'

DON'T:

Don't add lineageTag property when creating new objects
Don't add comments to the TMDL code. TMDL language don't support // comments. It does support descriptions (see Task: Setting descriptions in TMDL objects). But comments can be within Power Query (M) expressions or DAX expressions code blocks.

Example of a TMDL of a semantic model:


model Model    
    culture: en-US  

    table Sales
        
        measure 'Total Amount' = SUM('Sales'[Amount])
            formatString: $ #,##0
    
        column 'Product Key'
            dataType: int64
            isHidden
            sourceColumn: ProductKey
            summarizeBy: None
    
        column Amount
            dataType: double
            isHidden
            sourceColumn: Amount
            summarizeBy: None

        ...

        partition 'Sales-Partition' = m
            mode: import
            source = 
                let
                    Source = Sql.Database(Server, Database)
                    …

    table Product
              
        column 'Product Key'
            dataType: int64
            isKey
            sourceColumn: ProductKey
            summarizeBy: none

        ...

        partition 'Product-Partition' = m
            mode: import
            source = 
                let
                    Source = Sql.Database(Server, Database),
                    …

    relationship cdb6e6a9-c9d1-42b9-b9e0-484a1bc7e123
        fromColumn: Sales.'Product Key'
        toColumn: Product.'Product Key'

TMDL scripts

TMDL scripts are produced by TMDL view and are normally under the TMDLScripts folder of a semantic model.

A TMDL script always include a command in the top followed by one or more objects with at least one level of indentation.

<TMDL Command name>
  <TMDL object>
  
  <TMDL object>

The semantics of TMDL language are applied to objects within the command
TMDL scripts only support one command today: createOrReplace

Example of a TMDL script of a table using createOrReplace command:

createOrReplace

    table Product

        measure '# Products' = COUNTROWS('Product')
            formatString: #,##0

        column 'Product Name'
            dataType: string

        ...

TMDL table examples by storage mode

Import table

table customer	

	column customerId
		dataType: int64				
		sourceColumn: customerId		

	column customer
		dataType: string								
		sourceColumn: customer

	partition customer = m
		mode: import
		source =
				let
				    ...
				in
				    #"Final Step"

Direct Lake table

expression DatabaseQuery =
        let
            Source = AzureStorage.DataLake("https://onelake.dfs.fabric.microsoft.com/[WORKSPACE_ID]/[LAKEHOUSE_ID]", [HierarchicalNavigation=true])
        in
            Source

table customer	

	column customerId
		dataType: int64				
		sourceColumn: customerId		

	column customer
		dataType: string								
		sourceColumn: customer

	partition customer = entity
		mode: directLake
		source
			entityName: customer
			schemaName: dbo
			expressionSource: DatabaseQuery

Task: Export semantic model database to code format

Use the MCP tool to export the semantic model as TMDL code. Make sure you respect the PBIP file structure, see pbip.md for more details.

This is useful to use the Fabric CLI to import to a workspace.

Task: Setting descriptions in TMDL objects

✅ DO:

The format should be '/// Description' placed right above each object such as 'table, 'column', or 'measure' identifier in the TMDL code.

/// Description line 1
/// Description line 2
measure 'Measure1' = [DAX Expression]
    formatString: #,##0

/// Description line 1
column 'Column1'
    formatString: #,##0
    dataType: string

Ensure comments provide clear explanations of the definitions and purpose of the table, column or measure, incorporating COMPANY business and data practices where applicable.
Enhance existing descriptions but use them as baseline.
Use concise and meaningful descriptions that align with the purpose of the measure or column.

❌ DON'T:

Don't use the description property:

  measure 'Measure1' = [DAX Expression]
      formatString: #,##0
      description: 'Description line 1 Description line 2'

Don't change any other property while inserting descriptions

Task: Creating measures in TMDL

Always include a formatString property appropriate for the measure type.
Always include a description following the rules from Setting descriptions in TMDL objects section.
Don't create measures for non aggregatable columns such as keys or descriptions. Unless they specify a summarizeBy property different than 'none'
Don't create complex DAX. Keep it simple, most of the times I'm just trying to save some time for basic stuff.
Multi-line DAX expression should be enclosed within ```
The DAX expression should appear after the measure name preceeded with the '=' sign.
If its a single line DAX expression add it immediately after the measure name and '=' sign.

Measures should go to the top of the table object, before any column declaration.

Example:

table TableName

    /// Description measure 1
    measure 'Measure 1' = [Single Line DAX]
        formatString: #,##0

    /// Description measure 2
    measure Measure2 = ```
            [DAX Expression line 1
            DAX Expression line 2]
            ```
        formatString: #,##0

    column 'Product Name'
        dataType: string            
    
    ...

Task: Create new Row Level Security (RLS) Roles

If possible analyze the patterns of existing roles
When creating new roles, never include the PBI_Id annotation

Task: Setting descriptions in Power Query / M code in partition expressions

You are an assistant to help Power Query developers comment their code.
Insert a comment above the code explaining what that piece of code is doing.
Do not start the comment with the word Step or a number
Do not copy code into the comment.
Keep the comments to a maximum of 225 characters.
Update the step name explaining what that piece of code is doing.
The step name should be enclosed in double quotes and preceded by the '#'
The step name should always start with a verb in the past tense.
The step name should have spaces between words.
Keep the step name to a maximum of 50 characters.

References

External references (request markdown when possible):

TMDL docs

powerbi-tmdl