LookML to KTX Semantic Layer

LookML views map to SL sources, measure: to measures, explore: { join: } to the join graph. This skill lays out the mapping and the three capture shapes.

Mapping table

Type map: date/datetime/timestamp → time; yesno → boolean; number → number; string → string. Ignore drill_fields: (UI only).

Decision rules

LookML writes target the run connection directly. Unlike Looker runtime ingestion, the LookML adapter is configured on the warehouse KTX connection, so do not look for targetWarehouseConnectionId and do not route through a mapping array.

Before any SL write, inspect the WorkUnit notes.

If notes contain:

[LOOKML SL WRITES DISALLOWED]
reason: lookml_connection_mismatch
...
[/LOOKML SL WRITES DISALLOWED]

this is a hard gate. The model's declared Looker connection: does not match the warehouse connection's configured expectedLookerConnectionName. Continue wiki extraction and context candidates. Do not call sl_write_source or sl_edit_source for that WorkUnit. The runner also removes those write tools for this WorkUnit; treat the missing tools as expected. Preserve the mismatch reason in any emit_unmapped_fallback you create.

When SL is allowed:

• Overlay when the view is a thin wrapper over a manifest table (sql_table_name: matches a manifest entry). Do not repeat base columns or grain.
• Standalone when the view uses derived_table: or sql_always_where:. sl_write_source rejects overlays whose name has no manifest entry; that error points here.
• Skip a view with only view:, sql_table_name:, and bare dimension: entries (no measure:, description:, derived_table:, sql_always_where:, join:). The pre-filter already short-circuits those.
• Include rawPaths on every sl_write_source/sl_edit_source call with the exact LookML raw file(s) that support the action.

Preflight: never guess column names

LookML's dimension_group: date { type: time; timeframes: [raw, date, week, month] } expands at Looker-render time into ${view.date_raw}, ${view.date_date}, ${view.date_week}, and so on. These are NOT physical warehouse columns. The physical column is whatever the group's sql: clause references (e.g. ${TABLE}.date → column date).

A prior replay hallucinated date_date, date_week into sql:, columns:, and grain: across 4+ standalones; every measure on each affected source returned 400 Unrecognized name: date_date at query time. Preventable.

Verify each sql_table_name from the LookML view with entity_details before mapping to an SL source.

Identifier Verification Protocol

Before writing a wiki page or SL source on any topic:

1. discover_data({query: "<topic>"}) - see what wikis, SL sources, and raw tables already exist. Prefer updating existing pages over creating new ones.

Before emitting any schema.table or schema.table.column into a wiki body, SL source, tables: frontmatter, sl_refs, or emit_unmapped_fallback:

2. entity_details({connectionId, targets: [{display: "<identifier>"}]}) - confirm the identifier resolves; inspect native types, FK/PK, and sampleValues.
3. For literal values from the source, such as status codes or plan tiers, check whether they appear in entity_details sampleValues for the relevant column. If sampleValues is short or the sample may have missed real values, run a sql_execution probe with the same warehouse connection id: sql_execution({connectionId, sql: "SELECT DISTINCT <col> FROM <ref> LIMIT 50"}).
4. If the candidate identifier still does not resolve, do one of:
   • Use sql_execution({connectionId, sql: "SELECT 1 FROM <ref> LIMIT 0"}). If it errors, the identifier is fictional.
   • Wrap the identifier in [unverified - from <rawPath>] in the wiki body, citing the exact raw path that mentioned it.
   • When recording emit_unmapped_fallback with no_physical_table, include the failing probe error in clarification.
5. Never copy <schema>.<table> placeholder strings from these instructions into output.

Required flow before writing any overlay or standalone:

1. Call sl_discover({ query: "<tableName>" }) for each base table you're about to touch. That returns the real columns.
2. If the table isn't in the manifest, use the warehouse connectionId returned by discover_data or the target connection chosen from sl_discover, then call a dialect-appropriate SQL probe with that connection id, for example: sql_execution({connectionId: "warehouse", sql: "SELECT 1 FROM analytics.orders LIMIT 0"}). Replace warehouse, analytics, and orders with the verified connection, schema or dataset, and table from the WorkUnit evidence.
3. Use only those names in sql:, columns:, and grain:. Map each dimension_group to ONE { name: <physical_col>, type: time, role: time } entry - never one per timeframe.

After every sl_write_source: call sl_validate. It runs SELECT * FROM (<your sql:>) LIMIT 0 against the connection. If a column name was invented, the warehouse's Unrecognized name: … error comes back verbatim. Treat that as a hard failure - re-read the real columns with sl_discover and rewrite.

Provenance markers

When a wiki mixes LookML source prose with sl_discover output, tag sections:

<!-- from: lookml -->
Customers fan out many-to-one into `accounts` via `account_id`.
<!-- /from -->
<!-- from: bq_schema -->
`customers.admin_user_id` is nullable - orphan rows exist.
<!-- /from -->

Invisible in most renderers; lets a future pass audit provenance.

Example 1 - overlay (thin wrapper)

LookML (excerpt):

view: fct_labs {
  sql_table_name: analytics.fct_labs ;;
  dimension: is_byol { type: yesno; sql: ${TABLE}.lab_type = 'byol' ;; }
  measure: count_lab_orders { type: count; description: "Total lab orders." }
  measure: count_byol_labs { type: count; filters: [is_byol: "yes"] }
}
explore: fct_labs {
  join: dim_customers { sql_on: ${fct_labs.admin_user_id} = ${dim_customers.admin_user_id} ;; relationship: many_to_one }
}

KTX overlay at <connId>/fct_labs.yaml:

name: fct_labs
descriptions:
  user: "Lab-order fact table. One row per lab order event."
columns:
  - name: is_byol
    type: boolean
    expr: "lab_type = 'byol'"
measures:
  - name: count_lab_orders
    expr: count(lab_order_id)
    description: Total lab orders.
  - name: count_byol_labs
    expr: count(lab_order_id)
    filter: "is_byol = true"
joins:
  - to: dim_customers
    on: "admin_user_id = dim_customers.admin_user_id"
    relationship: many_to_one

Example 2 - standalone from derived_table

view: lab_results {
  derived_table: { sql:
    SELECT lab_order_id, admin_user_id, lab_date, biomarker, value,
           value - LAG(value) OVER (PARTITION BY admin_user_id, biomarker ORDER BY lab_date) AS delta
    FROM analytics.raw_lab_results WHERE status = 'final' ;; }
  dimension: lab_order_id { primary_key: yes; type: string }
  measure: avg_delta { type: average; sql: ${delta} ;; }
}

name: lab_results
description: "Lab results with biomarker delta vs previous reading per user."
source_type: sql
sql: |
  SELECT lab_order_id, admin_user_id, lab_date, biomarker, value,
         value - LAG(value) OVER (PARTITION BY admin_user_id, biomarker ORDER BY lab_date) AS delta
  FROM analytics.raw_lab_results WHERE status = 'final'
grain: [lab_order_id]
columns:
  - { name: lab_order_id, type: string }
  - { name: admin_user_id, type: string }
  - { name: lab_date, type: time, role: time }
  - { name: biomarker, type: string }
  - { name: value, type: number }
  - { name: delta, type: number }
measures:
  - { name: count_lab_results, expr: "count(lab_order_id)" }
  - { name: avg_delta, expr: "avg(delta)" }

Example 3 - standalone with sql_always_where

view: rpt_daily_braze_email {
  sql_table_name: analytics.fct_email_sends ;;
  sql_always_where: ${TABLE}.channel = 'braze' AND ${TABLE}.status = 'delivered' ;;
  dimension: send_id { primary_key: yes; type: string }
  measure: delivered_count { type: count }
}

name: rpt_daily_braze_email
description: "Delivered Braze email sends (enforced filter: channel='braze', status='delivered')."
source_type: sql
sql: |
  SELECT * FROM analytics.fct_email_sends
  WHERE channel = 'braze' AND status = 'delivered'
grain: [send_id]
columns:
  - { name: send_id, type: string }
  - { name: admin_user_id, type: string }
  - { name: sent_at, type: time, role: time }
measures:
  - { name: delivered_count, expr: "count(send_id)" }

sql_always_where is enforcement → wrap into the sql:. Don't model it as a segment (segments are opt-in) or per-measure filter (fragile, duplicated).