Running a snapshot with missing required configurations results in uncaught Python exception

[jnatkins]

Describe the bug

In an attempt to educate myself on how snapshots work, I created a snapshot to test out, but neglected to realize that the configuration object was required. When I ran dbt snapshot, it failed with a fairly non-actionable error message:

2021-05-20T22:41:00.291112Z: uncaught python exception
'NoneType' object has no attribute 'lower' Code: -32000

Steps To Reproduce

1. Create a snapshot without a config object, i.e.

{% snapshot snap_demo %}

select * from {{ ref('snapshot_demo_source') }}

{% endsnapshot %}

2. Run dbt snapshot

Expected behavior

It would be helpful to get an error message that indicates what actually went wrong, and guides the user to provide required configurations

Screenshots and log output

If applicable, add screenshots or log output to help explain your problem.

System information

Which database are you using dbt with?

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

The output of dbt --version:

0.19.1

The operating system you're using:
MacOS

The output of python --version:

Additional context

Add any other context about the problem here.

[jtcohen6]

@jnatkins Such a good issue, and a great catch, thank you for opening!

I think there's a pretty straightforward resolution here, which is adding a check to the validate method of SnapshotConfig:

https:/fishtown-analytics/dbt/blob/e775f2b38e52e8b9f25f8ac98c942d22b2c8c55d/core/dbt/contracts/graph/model_config.py#L448-L485

This method already exists to ensure that snapshots with certain strategies (timestamp, check) also contain the required associated configurations. We should first just check to make sure the three required configurations are present. Something like:

        if not data.get('strategy') or not data.get('unique_key') or not data.get('target_schema'):
            raise ValidationError(
                "Snapshots must be configured with a 'strategy', 'unique_key', and 'target_schema'.")

So instead of 'NoneType' object has no attribute 'lower', you'd get:

Compilation Error in snapshot my_snap (snapshots/my_snap.sql)
  at path []: Snapshots must be configured with a 'strategy', 'unique_key', and 'target_schema'.

Whether those configs are set via in-file config() blocks, or in dbt_project.yml, doesn't much matter. They just need to be set one way or another.

I'm going to tag this as a good first issue. Is this a fix you (or anyone else) might be interested in contributing? :)

[jnatkins]

@jtcohen6 I'd definitely be interested in contributing a fix here. I don't have a dev environment set up for dbt core yet, but I can probably scrape something together pretty quickly.