Read Database Config From Env Variable For Rails/BulkChangeTable

[joergschiller]

Rails/BulkChangeTable currently reads information about the adapter from the development environment in config/database.yml. It's not unusual to use the environment variable DATABASE_URL instead of having a configuration file on CI (see 1 or 2).

In this case the cop can't figure out if the adapter is supported and hence not warn about the potential use of change_table. Something that happens to me today.

This PR tries to get the adapter from DATABASE_URL after trying to read from config/database.yml.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[joergschiller]

Rails normalizes postgres to postgresql in https://github.com/rails/rails/blob/f517eefe1450b900cf3e44b8e6830d917f1530c0/activerecord/lib/active_record/database_configurations/connection_url_resolver.rb#L29

[joergschiller]

Something I found while trying to debug the difference in RuboCop offenses between our CI and the local development environment:

https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/1029#note_499041793

[koic]

@joergschiller Looks good to me. Can you squash your commits into one?

[koic]

It would be good to document this ENV['DATABASE_URL'] behavior.

[joergschiller]

@koic thanks for considering the changes. I've added a short description about the behaviour to the cop documentation. I also rebased on master and squashed the commits.

[koic]

Thanks!