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bsl-model-builder

@boringdata/boring-semantic-layer
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Build BSL semantic models with dimensions, measures, joins, and YAML config. Use for creating/modifying data models.

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SKILL.md

name bsl-model-builder
description Build BSL semantic models with dimensions, measures, joins, and YAML config. Use for creating/modifying data models.

BSL Model Builder

You are an expert at building semantic models using the Boring Semantic Layer (BSL).

Core Concepts

A Semantic Table transforms a raw Ibis table into a reusable data model:

  • Dimensions: Attributes to group by (categorical data)
  • Measures: Aggregations and calculations (quantitative data)

Creating a Semantic Table

from boring_semantic_layer import to_semantic_table

# Start with an Ibis table
flights_st = to_semantic_table(flights_tbl, name="flights")

with_dimensions()

Define groupable attributes using lambda, unbound syntax (_.), or Dimension class:

from ibis import _
from boring_semantic_layer import Dimension

flights_st = flights_st.with_dimensions(
    # Lambda - explicit
    origin=lambda t: t.origin,

    # Unbound syntax - concise
    destination=_.dest,
    year=_.year,

    # Dimension class - with description (AI-friendly)
    carrier=Dimension(
        expr=lambda t: t.carrier,
        description="Airline carrier code"
    )
)

Time Dimensions

Use .truncate() for time-based groupings:

flights_st = flights_st.with_dimensions(
    # Year, Quarter, Month, Week, Day
    arr_year=lambda t: t.arr_time.truncate("Y"),
    arr_month=lambda t: t.arr_time.truncate("M"),
    arr_date=lambda t: t.arr_time.truncate("D"),
)

Truncate units: "Y" (year), "Q" (quarter), "M" (month), "W" (week), "D" (day), "h", "m", "s"

with_measures()

Define aggregations using lambda or Measure class:

from boring_semantic_layer import Measure

flights_st = flights_st.with_measures(
    # Simple aggregations
    flight_count=lambda t: t.count(),
    total_distance=lambda t: t.distance.sum(),
    avg_delay=lambda t: t.dep_delay.mean(),
    max_delay=lambda t: t.dep_delay.max(),

    # Composed measures (reference other measures)
    avg_distance_per_flight=lambda t: t.total_distance / t.flight_count,

    # Measure class - with description
    avg_distance=Measure(
        expr=lambda t: t.distance.mean(),
        description="Average flight distance in miles"
    )
)

Percent of Total with all()

Use t.all() to reference the entire dataset:

flights_st = flights_st.with_measures(
    flight_count=lambda t: t.count(),
    market_share=lambda t: t.flight_count / t.all(t.flight_count) * 100
)

Joins

join_many() - One-to-Many (LEFT JOIN)

# One carrier has many flights
flights_with_carriers = flights_st.join_many(
    carriers_st,
    lambda f, c: f.carrier == c.code
)

join_one() - One-to-One (INNER JOIN)

# Each flight has exactly one carrier
flights_with_carrier = flights_st.join_one(
    carriers_st,
    lambda f, c: f.carrier == c.code
)

join_cross() - Cartesian Product

all_combinations = flights_st.join_cross(carriers_st)

Custom Joins

flights_st.join(
    carriers_st,
    lambda f, c: f.carrier == c.code,
    how="left"  # "inner", "left", "right", "outer", "cross"
)

After joins: Fields are prefixed with table names (e.g., flights.origin, carriers.name)

Multiple joins to same table: Use .view() to create distinct references:

pickup_locs = to_semantic_table(locs_tbl.view(), "pickup_locs")
dropoff_locs = to_semantic_table(locs_tbl.view(), "dropoff_locs")

YAML Configuration

Define models in YAML for better organization:

# flights_model.yaml
profile: my_db  # Optional: use a profile for connections

flights:
  table: flights_tbl
  dimensions:
    origin: _.origin
    destination: _.dest
    carrier: _.carrier
    arr_year: _.arr_time.truncate("Y")
  measures:
    flight_count: _.count()
    total_distance: _.distance.sum()
    avg_distance: _.distance.mean()

carriers:
  table: carriers_tbl
  dimensions:
    code: _.code
    name: _.name
  measures:
    carrier_count: _.count()

YAML uses unbound syntax only (_.field), not lambdas.

Loading YAML Models

from boring_semantic_layer import from_yaml

# With profile (recommended)
models = from_yaml("flights_model.yaml")

# With explicit tables
models = from_yaml(
    "flights_model.yaml",
    tables={"flights_tbl": flights_tbl, "carriers_tbl": carriers_tbl}
)

flights_sm = models["flights"]

Best Practices

  1. Add descriptions to dimensions/measures for AI-friendly models
  2. Use meaningful names that reflect business concepts
  3. Define composed measures to avoid repetition
  4. Use YAML for production models (version control, collaboration)
  5. Use profiles for database connections (see Profile docs)

Common Patterns

Derived Dimensions

flights_st = flights_st.with_dimensions(
    # Extract from timestamp
    arr_year=lambda t: t.arr_time.truncate("Y"),
    arr_month=lambda t: t.arr_time.truncate("M"),

    # Categorize numeric values (use ibis.cases - PLURAL, not ibis.case)
    distance_bucket=lambda t: ibis.cases(
        (t.distance < 500, "Short"),
        (t.distance < 1500, "Medium"),
        else_="Long"
    )
)

Ratio Measures

flights_st = flights_st.with_measures(
    total_flights=lambda t: t.count(),
    delayed_flights=lambda t: (t.dep_delay > 0).sum(),
    delay_rate=lambda t: t.delayed_flights / t.total_flights * 100
)

Additional Information

Available documentation: