Using Cube with dbt

dbt (opens in a new tab) is a popular data integration tool used by many Cube users to transform the data in the data warehouse before bringing it to the semantic layer.

Usually, dbt projects are focused (opens in a new tab) on creating staging and intermediate models and finally using them to define normalized data marts (opens in a new tab) for the semantic layer. Often, these marts would be described in .yml files as models (opens in a new tab) with columns (opens in a new tab) and other properties.

Cube projects usually define cubes on top of dbt models, materialized as tables or views in your data warehouse, bring columns as dimensions, enrich them with measures, joins, and pre-aggregations. Finally, the facade of the data model is created with views and exposed to downstream tools via a rich set of APIs.

cubes:
  - name: orders
  - sql_table: my_dbt_schema.my_dbt_orders_model
 
    dimensions: []
    measures: []
    joins: []
    pre_aggregations: []

Cube provides a convenient cube_dbt package to simplify data model integration.

Data transformation

Data types

Cube provides a single time type to model time dimensions. However, raw data can contain temporal columns in various formats, e.g., as TIMESTAMP, DATETIME, DATE, etc. It's a best practice to convert all temporal columns in dbt models into the TIMESTAMP format.

Refresh keys

It's convenient to have an updated_at column in every dbt model so that the refresh keys of cubes would be defined as follows:

SELECT MAX(updated_at)
FROM orders

When the source data doesn't have a column that can be used to track updates reliably and dbt snapshots (opens in a new tab) are used, dbt_valid_from and dbt_valid_to columns can be used to define refresh keys, e.g.:

SELECT CONCAT(COUNT(*), MAX(dbt_valid_from), MIN(dbt_valid_to))
FROM orders_snapshot

Pre-aggregations

When pre-aggregations are implemented, they would be refreshed using their own refresh keys.

If applicable, consider configuring a data orchestration tool like Airflow, Dagster, or Prefect so that it triggers pre-aggregation refresh when dbt models are updated via the Orchestration API.

Data model integration

cube_dbt package simplifies defining the data model on top of dbt models. It provides convenient tools for loading the metadata of a dbt project, inspecting dbt models, and rendering them as cubes with dimensions. It's designed to work with dynamic data models built with YAML, Jinja, and Python.

Usually, the integration would include the following steps:

• Load the metadata of your dbt project.
• Select relevant dbt models.
• Render dbt models as cubes with dimensions.
• Enrich these cubes with measures, joins, pre-aggregations, etc.

Installation

Add the cube_dbt package to the requirements.txt file in the root directory of your Cube project. See the cube_dbt package documentation for details.

Loading dbt projects

cube_dbt package loads the metadata of dbt projects from the manifest.json file (opens in a new tab). Any dbt command that parses the dbt project would produce this file, including dbt build and dbt run.

In the most basic scenario, you can commit the manifest.json file to the source code of your Cube project and load it as a local file. Here's an example source code of the globals.py file in your data model folder:

from cube_dbt import Dbt
 
dbt = Dbt.from_file('./manifest.json')

In a less basic scenario, you can put the manifest.json file to a remote storage and load it by its URL:

from cube_dbt import Dbt
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
dbt = Dbt.from_url(manifest_url)

Actually, you can obtain the contents of the manifest.json file in any way that works for you. parse the JSON into a dictionary, and load it directly. This is particularly useful when loading manifest.json from non-public remote storage using packages like smart-open (opens in a new tab) or boto3 (opens in a new tab).

from cube_dbt import Dbt
from smart_open import open
import json
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
manifest = None
 
with open(manifest_url) as file:
  manifest_json = file.read()
  manifest = json.loads(manifest_json)
 
dbt = Dbt(manifest)

Selecting dbt models

By default, cube_dbt would load all dbt models, excluding ephemeral (opens in a new tab) ones. You can also cherry-pick dbt models and load only some of them.

For example, you can load only models under a directory or directories. This is particularly useful if you keep your data marts under models/marts/ as advised by dbt (opens in a new tab).

from cube_dbt import Dbt
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
dbt = Dbt.from_url(manifest_url).filter(paths=['marts/'])

You can also load only models with certain tags. This is useful if you'd like to have more granular controls over dbt models exposed to the semantic layer:

from cube_dbt import Dbt
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
dbt = Dbt.from_url(manifest_url).filter(tags=['cube'])

Finally, you can load only select models by their names:

from cube_dbt import Dbt
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
dbt = Dbt.from_url(manifest_url).filter(names=['orders', 'customers'])

Rendering dbt models

cube_dbt provides convenient tools to render dbt models in YAML files with Jinja. For that, dbt models should be made accessible from Jinja templates. Here, they are exposed via dbt_models and dbt_model functions:

from cube import TemplateContext
from cube_dbt import Dbt
 
manifest_url = 'https://cube-dbt-integration.s3.amazonaws.com/manifest.json'
dbt = Dbt.from_url(manifest_url).filter(paths=['marts/'])
 
template = TemplateContext()
 
@template.function('dbt_models')
def dbt_models():
  return dbt.models
 
@template.function('dbt_model')
def dbt_model(name):
  return dbt.model(name)

You can use the dbt_models function in a Jinja template to iterate over all models and output them as YAML. Note the convenient as_cube and as_dimensions methods that would render each model with correct name and sql_table and also render each column as a dimension.

cubes:
  {% for model in dbt_models() %}
  - {{ model.as_cube() }}
 
    dimensions:
      {{ model.as_dimensions(skip=['very_private_column']) }}
 
    measures:
      - name: count
        type: count
  {% endfor %}

cube_dbt does its best to translate data_type properties of columns to correct dimension types. Also, if a column is tagged as primary_key in its properties, it would automatically become a primary key dimension. You can also prevent select columns from being rendered as dimensions with skip.

You can even iterate over columns on your own and render them as you wish:

cubes:
  {% for model in dbt_models() %}
  - {{ model.as_cube() }}
 
    dimensions:
      {% for column in model.columns() %}
      - name: "{{ column.name }}"
        sql: "{{ column.sql }}"
        type: "{{ column.type }}"
        meta:
          source: dbt
      {% endfor %}
 
    measures:
      - name: count
        type: count
  {% endfor %}

However, in a real-world scenario, iterating over all dbt models and rendering them in a single YAML file is inconvenient since it would be difficult to enrich each rendered cube with its own measures, joins, etc.

Enriching the data model

It's best practice to render each dbt model (or data mart) as cube in its own file. It simplifies Jinja templates and improves the maintainability of the data model. Consider the following cube:

{% set model = dbt_model('orders') %}
 
cubes:
  - {{ model.as_cube() }}
 
    dimensions:
      {{ model.as_dimensions() }}
 
    # Model-specific measures
    measures:
      - name: count
        type: count
 
      - name: statuses
        sql: "STRING_AGG({status})"
        type: string
 
    # Model-specific joins
    joins:
      - name: users
        sql: "{CUBE.user_id} = {users.id}"
        relationship: many_to_one
 
    # Model-specific pre-aggregations
    pre_aggregations:
      - name: count_by_users
        measures:
          - CUBE.count
        dimensions:
          - users.full_name

You can easily customize measures, joins, and pre-aggregation that apply to this very cube, mapped to the orders dbt model, and have it side-by-side with other cubes, even of they don't come from dbt.

To inspect rendered cubes, you can use Data Graph, Playground, or the meta endpoint of the REST API.