persist_docs functionality doesn't persist descriptions on nested bigquery columns

[bodschut]

Describe the bug

With the release of dbt 0.17.0, the persist_docs functionality was enhanced to also apply column descriptions to bigquery when they are supplied in a schema.yml file. However, it seems that it only takes into account top-level columns and there is no way to persist a description to a nested column (ie a field that is part of a RECORD type bigquery column).

Steps To Reproduce

Suppose you create a simple test model using dbt in bigquery:

SELECT
    STRUCT(
        STRUCT(
            1 AS level_3_a,
            2 AS level_3_b
        ) AS level_2
    ) AS level_1

This will create a view/table in bigquery with the following schema:

We can describe this table in a schema.yml file and add descriptions as follows:

version: 2

models:
  - name: test_nested
  
    columns:
      - name: level_1
        description: level_1 description
      - name: level_1.level_2.level_3_a
        description: level_3_a description

When we now perform a dbt run on this model, only the description on the top level field is added and no warnings or errors are generated by dbt:

Expected behavior

When providing descriptions for nested fields (preferably using the dotted syntax as shown above), the descriptions should be correctly persisted on nested fields.

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

installed version: 0.17.0
   latest version: 0.17.0

Up to date!

Plugins:
  - snowflake: 0.17.0
  - postgres: 0.17.0
  - bigquery: 0.17.0
  - redshift: 0.17.0

The operating system you're using:
Mac OS 10.15.3

The output of python --version:
Python 3.7.5

[bodschut]

Should we close this issue @beckjake ?

[drewbanin]

closed by #2550

Thanks for your contribution @bodschut!