Fix case when we want a UrlConfig but the URL is nil

Previously if the `url` key in a config hash was nil we'd ignore the
configuration as invalid. This can happen when you're relying on a
`DATABASE_URL` in the env and that is not set in the environment.

```
production:
  <<: *default
  url: ENV['DATABASE_URL']
```

This PR fixes that case by checking if there is a `url` key in the
config instead of checking if the `url` is not nil in the config.

In addition to changing the conditional we then need to build a url hash
to merge with the original hash in the `UrlConfig` object.

Fixes #35091

1 files changed, 3 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/database_configurations.rb b/activerecord/lib/active_record/database_configurations.rb
index 11aed6c002..73adf66684 100644
--- a/activerecord/lib/active_record/database_configurations.rb
+++ b/activerecord/lib/active_record/database_configurations.rb
@@ -134,9 +134,11 @@ module ActiveRecord
       end
 
       def build_db_config_from_hash(env_name, spec_name, config)
-        if url = config["url"]
+        if config.has_key?("url")
+          url = config["url"]
           config_without_url = config.dup
           config_without_url.delete "url"
+
           ActiveRecord::DatabaseConfigurations::UrlConfig.new(env_name, spec_name, url, config_without_url)
         elsif config["database"] || (config.size == 1 && config.values.all? { |v| v.is_a? String })
           ActiveRecord::DatabaseConfigurations::HashConfig.new(env_name, spec_name, config)