---
name: dbt_create_mart
description: "Step-by-step instructions for creating dbt Mart models (facts and dimensions) as thin wrappers over transform models. Use for any dbt + Snowflake project."
---

# Create a dbt Mart Model (Fact or Dimension)

Mart models are **thin wrappers** over transform models. They expose clean, BI-ready tables without duplicating business logic.

---

## Fact Model Steps

**1. Name the file**
- Pattern: `fct_<domain>_<metric>.sql`
- Example: `fct_sales_orders.sql`, `fct_ar_open_items.sql`
- Lives in `models/marts/facts/`

**2. Write a thin SELECT wrapper**

Option A — expose all columns (most common):
```sql
select {{ dbt_utils.star(ref('<trf_model_name>')) }}
from {{ ref('<trf_model_name>') }}
```

Option B — add a light filter or derived column:
```sql
select
    *,
    <derived_column> as <alias>
from {{ ref('<trf_model_name>') }}
where <filter_condition>
```

**3. Materialization**
- Default: `view` (inherited from `dbt_project.yml`).
- Override to `table` only if the BI tool performs poorly on views.

---

## Dimension Model Steps

**1. Name the file**
- Pattern: `dim_<entity>.sql`
- Example: `dim_customers.sql`, `dim_materials.sql`
- Lives in `models/marts/dim/`

**2. Write the SELECT**
- Dimensions are usually deduped/distinct entity lists from staging or transform.
```sql
select distinct
    <natural_key>          as <entity>_key,
    <descriptive_field_1>,
    <descriptive_field_2>
from {{ ref('<stg_or_trf_model>') }}
where <natural_key> is not null
```

**3. Add schema entry**
- File: `models/marts/schema.yml`
- Every fact: test `unique` + `not_null` on the primary key.
- Every dimension: test `unique` + `not_null` on the surrogate/natural key.
- Add `relationships` tests on foreign keys in fact models.

---

## Rules

| Rule | Detail |
|------|--------|
| No business logic | Mart = wrapper only; logic belongs in transform |
| Materialization | `view` by default; `table` only if needed for BI performance |
| Column naming | `snake_case` for marts; inherit from the transform model |
| `ref()` only | Always reference the transform model with `{{ ref() }}` |
| BI-facing names | Rename columns here if the BI layer needs friendlier names |

---

## Templates

See `mart_fct_template.sql` and `mart_dim_template.sql` in this folder.