Describe/explain why different Jinja features are/aren't available in different contexts

[joellabes]

This should answer questions like:

• "Why can't I use a var in a var in dbt_project.yml?"
• "Why can I do materialized: "{{ 'table' if target.name == 'prod' else 'view' }}" in dbt_project.yml but not
  materialized: {{ table_if_prod() }}?"
• Why does {{ this }} sometimes evaluate to None?

With answers like

• vars are defined inside dbt_project.yml, so you can't access them until after the whole file has been evaluated, which is too late.
• Because target is evaluated before dbt_project.yml early and macros aren't available until after
• I don't know offhand if this is true, but I bet it is and good lists come in threes. Maybe ephemeral models?

It would be useful to show the order of parsing, describe the concepts, and show a matrix of where things can be used.

I think this is a matrix of

Different tools

• var
• this
• target
• custom macros
• ...

vs different contexts

• dbt_project.yml
• config() block
• inside a model's body
• ...

[runleonarun]

@joellabes Is there someone or a group of someones we should @mention to get this discussion rolling?

[joellabes]

Unfortunately for his inbox, I think @jtcohen6 is probably still the person with the canonical list in his head 😬

[jtcohen6]

The canonical canonical list is: https://github.com/dbt-labs/dbt-core/tree/main/core/dbt/context

There's an inline comment here that's helpful, but really we need a README for this submodule! Then we could link to it from this docs page? (cc @dbt-labs/core-language)