How to parse a variable as a source reference in dbt?

Question

I am building a model where I am dynamically referencing the table name and schema name based on the results of a query.

    {%- set query %}
        select * from master_data.event_metadata where game='{{game_name}}'
    {% endset -%}
    {%- set results = run_query(query) -%}
    {%- if execute %}
        {%- set record = results.rows[0] -%}
    {% else %}
        {%- set record = [] -%}
    {% endif -%}

Two of the values are in record.SCHEMA_NAME and record.TABLE_NAME. I can use something like

select
    *
from
    {{record.SCHEMA_NAME}}.{{record.TABLE_NAME}}

but I'd rather use the source() function instead so that my documentation and DAG will be clean. How can I parse record.SCHEMA_NAME and record.TABLE_NAME as string arguments. I need to have something like

select
    *
from
    {{ source(record.SCHEMA_NAME, record.TABLE_NAME) }}

When I try to run the above I get the below error:

Server error: Compilation Error in rpc request (from remote system)
The source name (first) argument to source() must be a string, got <class 'jinja2.runtime.Undefined'>

gasscoelho gasscoelho · Accepted Answer · 2021-06-08T15:12:42

You might already have found a workaround or a solution for this, but just in case someone else comes to the same situation...

To convert the values to string you can use the |string. For instance:

record.SCHEMA_NAME|string
record.TABLE_NAME|string

So your query would look something like this:

select * from {{ source(record.SCHEMA_NAME|string|lower, record.TABLE_NAME|string|lower) }}

Note that depending on the output for your query and how you defined the source file, you might have to lower or upper your values to match with your source.

Problem

Your record variable is a result of an execution (run_query(query)). When you do dbt compile/run dbt will do a series of operations like read all the files of your project, generate a "manifest.json" file, and will use the ref/source to generate the DAG so at this point, no SQL is executed, in other words, execute == False.

In your example, even if you do record.SCHEMA_NAME|string you will not be able to retrieve the value of that variable because nothing was executed and since you did if not execute then record = [] , you will get that message ... depends on a source named '.' which was not found, because at that point, record is empty.

A workaround would be to wrap your model's query in a if execute block, something like this:

{% if execute %}
select * from {{ source(record.TABLE_SCHEMA|string|lower, record.TABLE_NAME|string|lower) }}
{% endif %}

With that approach, you will be able to dynamically set the source of your model.

But unfortunately, this will not work as you expected because it will not generate the DAG for that model. Using an if execute block to wrap your model's query will prevent dbt to generate the DAG for the model.

In the end, this would be the same as your first attempt on having the schema and table declared without the source function.

For more details, you can check the dbt documentation about the execute mode: https://docs.getdbt.com/reference/dbt-jinja-functions/execute

How to parse a variable as a source reference in dbt?

2 Answers

Problem