powerbi-power-query

Power Query for Semantic Models

Author, validate, and test Power Query M expressions in semantic model import partitions. Covers writing correct M code, preserving query folding, validating expressions, and testing them by executing against real data sources.

Partition Expressions

Each import table in a semantic model has a partition with an M expression defining what data gets loaded during refresh. The expression typically connects to a data source, navigates to a table/view, and applies transformations.

Structure of a Partition Expression

let
    Source = Sql.Database(#"SqlEndpoint", #"Database"),
    Data = Source{[Schema="dbo", Item="Orders"]}[Data],
    #"Removed Columns" = Table.RemoveColumns(Data, {"InternalId"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Removed Columns", {{"Amount", Currency.Type}})
in
    #"Changed Type"

Key elements:

• Parameters: #"SqlEndpoint", #"Database" are shared M parameters defined at the model level
• Navigation: Source{[Schema="dbo", Item="Orders"]}[Data] navigates to a specific table
• Steps: Each step is a named variable in the let...in chain
• Quoted identifiers: Step names with spaces use #"Step Name" syntax

Extracting Expressions

# Get partition expression from TMDL via fab
fab get "<Workspace>.Workspace/<Model>.SemanticModel" -f \
  -q "definition.parts[?path=='definition/tables/<Table>.tmdl'].payload"

# Get shared M parameters
fab get "<Workspace>.Workspace/<Model>.SemanticModel" -f \
  -q "definition.parts[?path=='definition/expressions.tmdl'].payload"

Writing M Expressions

Query Folding

Query folding is the most important performance concept. The M engine translates compatible steps into native data source queries (e.g., SQL). When folding breaks, subsequent steps run in the mashup engine, pulling all data into memory first.

Steps that typically fold (for SQL sources):

• Table.SelectColumns / Table.RemoveColumns -> SELECT
• Table.SelectRows -> WHERE
• Table.Sort -> ORDER BY
• Table.FirstN -> TOP
• Table.Group -> GROUP BY
• Table.RenameColumns -> AS aliases