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Dynamic data models with Jinja and Python

Dynamic data models with Jinja and Python

Cube supports authoring dynamic data models using the Jinja templating language (opens in a new tab) and Python. This allows de-duplicating common patterns in your data models as well as dynamically generating data models from a remote data source.

Jinja is supported in all YAML data model files.

Jinja

Please check the Jinja documentation (opens in a new tab) for details on Jinja syntax.

Loops

Jinja supports looping (opens in a new tab) over lists and dictionaries. In the following example, we loop over a list of nested properties and generate a LEFT JOIN UNNEST clause for each one: for each one:

{%- set nested_properties = [
  "referrer",
  "href",
  "host",
  "pathname",
  "search"
] -%}
 
cubes:
  - name: analytics
    sql: >
      SELECT
      {%- for prop in nested_properties %}
        {{ prop }}_prop.value AS {{ prop }}
      {%- endfor %}
      FROM public.events
      {%- for prop in nested_properties %}
      LEFT JOIN UNNEST(properties) AS {{ prop }}_prop ON {{ prop }}_prop.key = '{{ prop }}'
      {%- endfor %}

Another useful pattern is to loop over a dictionary of values and generate a measure for each one, as in the following example:

{%- set metrics = {
  "mau": 30,
  "wau": 7,
  "day": 1
} %}
 
cubes:
  - name: orders
    sql_table: public.orders
 
    measures:
      {%- for name, days in metrics.items() %}
      - name: {{ name }}
        type: count_distinct
        sql: user_id
        rolling_window:
          trailing: {{ days }} day
          offset: start
      {% endfor %}

Macros

Cube data models also support Jinja macros, which allow you to define reusable snippets of code. You can read more about macros in the Jinja documentation (opens in a new tab).

In the following example, we define a macro called dimension() which generates a dimension definition in Cube. This macro is then invoked multiple times to generate multiple dimensions:

{# Declare the macro before using it, otherwise Jinja will throw an error. #}
{%- macro dimension(column_name, type='string', primary_key=False) -%}
      - name: {{ column_name }}
        sql: {{ column_name }}
        type: {{ type }}
        {% if primary_key -%}
        primary_key: true
        {% endif -%}
{% endmacro -%}
 
cubes:
  - name: orders
    sql_table: public.orders
 
    dimensions:
      {{ dimension('id', 'number', primary_key=True) }}
      {{ dimension('status') }}
      {{ dimension('created_at', 'time') }}
      {{ dimension('completed_at', 'time') }}

You could also use macros to generate SQL snippets for use in the sql property:

{%- macro cents_to_dollars(column_name, precision=2) -%}
  ({{ column_name }} / 100)::NUMERIC(16, {{ precision }})
{%- endmacro -%}
 
cubes:
  - name: payments
    sql: >
      SELECT
        id AS payment_id,
        {{ cents_to_dollars('amount') }} AS amount_usd
      FROM app_data.payments

Escaping unsafe strings

Auto-escaping (opens in a new tab) of unsafe string values in Jinja templates is enabled by default. It means that any strings coming from Python might get wrapped in quotes, potentially breaking YAML syntax.

You can work around that by using the safe Jinja filter (opens in a new tab) with such string values:

cubes:
  - name: my_cube
    description: {{ get_unsafe_string() | safe }}

Alternatively, you can wrap unsafe strings into instances of the following class in your Python code, effectively marking them as safe. This is particularly useful for library code, e.g., similar to the cube_dbt package.

class SafeString(str):
  is_safe: bool
 
  def __init__(self, v: str):
    self.is_safe = True

Python

You can declare and invoke Python functions from within a Jinja template. This allows the reuse of existing code to generate data models. Cube uses Python 3.9 to execute Python code. It also installs packages listed in the requirements.txt with pip on the startup.

These helper functions must be located in model/globals.py file or explicitly loaded from the YAML files. In the following example, we declare a function called load_data() which will load data from a remote API endpoint. We will then use the function to generate a data model in Cube.

from cube import TemplateContext
 
template = TemplateContext()
 
@template.function('load_data')
def load_data():
   client = MyApiClient("example.com")
   return client.load_data()
 
 
class MyApiClient:
  def __init__(self, api_url):
    self.api_url = api_url
 
  # mock API call
  def load_data(self):
    api_response = {
      "cubes": [
        {
          "name": "cube_from_api",
          "measures": [
            { "name": "count", "type": "count" },
            { "name": "total", "type": "sum", "sql": "amount" }
          ],
          "dimensions": []
        },
        {
          "name": "cube_from_api_with_dimensions",
          "measures": [
            { "name": "active_users", "type": "count_distinct", "sql": "user_id" }
          ],
          "dimensions": [
            { "name": "city", "sql": "city_column", "type": "string" }
          ]
        }
      ]
    }
    return api_response

Now that we've decorated our function with the @template.function decorator, we can call it from within a Jinja template. In the following example, we'll call the load_data() function and use the result to generate a data model.

cubes:
  {# Here we use the decorated function from earlier #}
  {%- for cube in load_data()["cubes"] %}
 
  - name: {{ cube.name }}
 
  {%- if cube.measures is not none and cube.measures|length > 0 %}
    measures:
      {%- for measure in cube.measures %}
      - name: {{ measure.name }}
        type: {{ measure.type }}
      {%- if measure.sql %}
        sql: {{ measure.sql }}
      {%- endif %}
      {%- endfor %}
  {%- endif %}
 
  {%- if cube.dimensions is not none and cube.dimensions|length > 0 %}
    dimensions:
      {%- for dimension in cube.dimensions %}
      - name: {{ dimension.name }}
        type: {{ dimension.type }}
        sql: {{ dimension.sql }}
      {%- endfor %}
  {%- endif %}
  {%- endfor %}