wren-generate-mdl

name	wren-generate-mdl
description	Generate a Wren MDL project by exploring a database with available tools (SQLAlchemy, database drivers, MCP connectors, or raw SQL). Guides agents through schema discovery, type normalization, and MDL YAML generation using the wren CLI. Use when: user wants to create or set up a new MDL, onboard a new data source, or scaffold a project from an existing database.
license	Apache-2.0
metadata	{"author":"wren-engine","version":"2.2"}

Generate Wren MDL — CLI Agent Workflow

Version check

Silently fetch https://raw.githubusercontent.com/Canner/wren-engine/main/skills/versions.json. Compare the wren-generate-mdl key with this skill's version (from the frontmatter above). If the remote version is newer, notify the user before proceeding:

A newer version of the wren-generate-mdl skill is available. Update with:

npx skills add Canner/wren-engine --skill wren-generate-mdl

The CLI auto-detects your installed agent. To target a specific one, add --agent <name> (e.g., claude-code, cursor, windsurf, cline).

Then continue with the workflow below regardless of update status.

---

Builds an MDL project by discovering database schema and converting it into Wren's YAML project format. The agent uses whatever database tools are available in its environment for introspection; the wren CLI handles type normalization, validation, and build.

For memory and query workflows after setup, see the wren-usage skill.

---

Prerequisites

• wren CLI installed (pip install wren-engine[<datasource>])
• A working database connection (credentials available to the agent)
• A wren profile configured (wren profile add) or connection info ready

---

Phase 0 — Detect existing project

Goal: If the current directory is already inside a wren project, let the user decide how to proceed.

Check whether wren_project.yml exists in the current working directory (or any parent up to the repository root). If found:

1. Tell the user that an existing wren project was detected and show its path.
2. Ask:
   • Reset — wipe the existing project (models/, views/, relationships.yml, instructions.md, and rebuild wren_project.yml) and regenerate from scratch in the same directory.
   • New path — keep the existing project untouched and choose a different directory for the new project. Ask the user for the new path, then wren context init --path <new_path> and continue from Phase 1 using that path.

If no existing project is detected, proceed directly to Phase 1.

---

Phase 1 — Establish connection and scope

Goal: Confirm the agent can reach the database and agree on scope with the user.

1. Verify connectivity using whichever tool is available:

   • If SQLAlchemy: engine.connect() test
   • If database driver: simple query like SELECT 1
   • If wren profile exists: wren profile debug to check config
   • If raw SQL via wren: wren --sql "SELECT 1" (requires profile or connection file)

2. Ask the user:

   • Which schema(s) or dataset(s) to include (skip if only one exists)
   • Whether to include all tables or a subset
   • The datasource type for wren (e.g., postgres, bigquery, snowflake) — needed for type normalization dialect

---

Phase 2 — Discover schema

Goal: Collect table names, column names, column types, and constraints.

Use whatever introspection method is available. Here are common approaches ranked by convenience:

Option A: SQLAlchemy (recommended if available)

from sqlalchemy import create_engine, inspect

engine = create_engine(connection_url)
inspector = inspect(engine)

tables = inspector.get_table_names(schema="public")

for table in tables:
    columns = inspector.get_columns(table, schema="public")
    # columns → [{"name": "id", "type": INTEGER(), "nullable": False, ...}]

    pk = inspector.get_pk_constraint(table, schema="public")
    # pk → {"constrained_columns": ["id"], "name": "orders_pkey"}

    fks = inspector.get_foreign_keys(table, schema="public")
    # fks → [{"constrained_columns": ["customer_id"],
    #          "referred_table": "customers",
    #          "referred_columns": ["id"]}]

Option B: Database-specific driver

• psycopg / asyncpg (Postgres): Query information_schema.columns and information_schema.table_constraints
• google-cloud-bigquery: client.list_tables(), client.get_table() → table.schema
• snowflake-connector-python: SHOW COLUMNS IN TABLE, SHOW PRIMARY KEYS IN TABLE
• clickhouse-driver: DESCRIBE TABLE, system.tables

Option C: Raw SQL via wren

If no driver is available but a wren profile is configured, query information_schema through wren itself:

wren --sql "SELECT table_name FROM information_schema.tables WHERE table_schema = 'public'" -o json
wren --sql "SELECT column_name, data_type FROM information_schema.columns WHERE table_name = 'orders'" -o json

Note: this goes through the MDL layer, so it only works if you already have a minimal MDL or if the database supports information_schema as regular tables. For bootstrapping from zero, Option A or B is preferred.

---

Phase 3 — Normalize types

Goal: Convert raw database types to wren-core-compatible types.

Python import (recommended for batch processing)

from wren.type_mapping import parse_type, parse_types

# Single type
normalized = parse_type("character varying(255)", "postgres")  # → "VARCHAR(255)"

# Batch — entire table at once
columns = [
    {"column": "id", "raw_type": "int8"},
    {"column": "name", "raw_type": "character varying"},
    {"column": "total", "raw_type": "numeric(10,2)"},
]
normalized_cols = parse_types(columns, dialect="postgres")
# Each dict now has a "type" key with the normalized value

CLI (if Python import not available)

Single type:

wren utils parse-type --type "character varying(255)" --dialect postgres
# → VARCHAR(255)

Batch (stdin JSON):

echo '[{"column":"id","raw_type":"int8"},{"column":"name","raw_type":"character varying"}]' \
  | wren utils parse-types --dialect postgres

---

Phase 4 — Scaffold and write MDL project

Goal: Create the YAML project structure.

Step 1 — Initialize project

wren context init --path /path/to/project

This creates:

project/
├── wren_project.yml
├── models/
├── views/
├── relationships.yml
└── instructions.md

IMPORTANT: catalog and schema in wren_project.yml

These are Wren Engine's internal namespace — they are NOT the database's native catalog or schema. Keep the defaults (catalog: wren, schema: public) unless you are intentionally configuring a multi-project namespace.

Your database's actual catalog/schema is specified per-model in table_reference (see Step 2). Do not copy database catalog/schema values into wren_project.yml.

Step 2 — Write model files

For each table, create a YAML file under models/. Use snake_case naming (the build step converts to camelCase automatically).

# models/orders/metadata.yml
name: orders
table_reference:
  catalog: ""           # database catalog (empty string if not applicable;
                        #   for DuckDB, use the DB file name without extension,
                        #   e.g. jaffle_shop.duckdb → catalog: jaffle_shop)
  schema: public        # database schema (this IS the DB schema)
  table: orders         # database table name
primary_key: order_id
columns:
  - name: order_id
    type: INTEGER
    not_null: true
  - name: customer_id
    type: INTEGER
  - name: total
    type: "DECIMAL(10, 2)"
  - name: status
    type: VARCHAR
    properties:
      description: "Order status: pending, shipped, delivered, cancelled"

Step 3 — Write relationships

From foreign key constraints discovered in Phase 2:

# relationships.yml
- name: orders_customers
  models:
    - orders
    - customers
  join_type: many_to_one
  condition: "orders.customer_id = customers.customer_id"

Join type mapping:

• FK table → PK table: many_to_one
• PK table → FK table: one_to_many
• Unique FK: one_to_one
• Junction table: many_to_many

If no foreign keys were found, infer from naming conventions:

• Column <table>_id or <table_singular>_id → likely FK to <table>
• Ask the user to confirm inferred relationships

Step 4 — Add descriptions (optional but valuable)

Ask the user to describe:

• Each model (1-2 sentences about what the table represents)
• Key columns (especially calculated fields or non-obvious names)

These descriptions are indexed by wren memory index and significantly improve LLM query accuracy.

---

Phase 5 — Validate and build

# Validate YAML structure and integrity
wren context validate --path /path/to/project

# If strict mode is desired:
wren context validate --path /path/to/project --strict

# Build JSON manifest
wren context build --path /path/to/project

# Verify against database
wren --sql "SELECT * FROM <model_name> LIMIT 1"

If validation fails, fix the reported issues and re-run. Common errors:

• Duplicate model/column names
• Missing primary key
• Relationship referencing non-existent model
• Invalid column type (try re-running through parse_type)

---

Phase 6 — Initialize memory

# Index schema (generates seed NL-SQL examples automatically)
wren memory index

# Verify
wren memory status

After this step, wren memory fetch and wren memory recall are operational. See the wren-usage skill for query workflows.

---

Phase 7 — Iterate with the user

The initial MDL is a starting point. Improve it by:

• Adding calculated columns based on business logic
• Adding views for common query patterns
• Refining descriptions based on actual query usage
• Adding access control (RLAC/CLAC) if needed

Each change follows: edit YAML → wren context validate → wren context build → wren memory index.

---

Quick reference

Task	Command / Method
Discover tables	Agent's own tools (SQLAlchemy, driver, raw SQL)
Discover columns + types	Agent's own tools
Discover constraints	Agent's own tools
Normalize types (Python)	from wren.type_mapping import parse_type
Normalize types (CLI)	wren utils parse-type --type T --dialect D
Normalize types (batch)	wren utils parse-types --dialect D < columns.json
Scaffold project	wren context init
Write models	Create models/<name>/metadata.yml
Write relationships	Edit relationships.yml
Validate	wren context validate
Build manifest	wren context build
Test query	wren --sql "SELECT * FROM <model> LIMIT 1"
Index memory	wren memory index

---

Things to avoid

• Do not hardcode database-specific type strings in MDL — always normalize via parse_type
• Do not skip validation before build — invalid YAML produces broken manifests silently
• Do not guess column types — introspect from the actual database
• Do not write relationships without confirming join conditions — wrong conditions cause silent query errors
• Do not skip wren memory index after build — stale indexes degrade recall quality