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, 8 insertions, 0 deletions

diff --git a/activerecord/test/cases/connection_adapters/merge_and_resolve_default_url_config_test.rb b/activerecord/test/cases/connection_adapters/merge_and_resolve_default_url_config_test.rb
index 06c1c51724..225cccc62c 100644
--- a/activerecord/test/cases/connection_adapters/merge_and_resolve_default_url_config_test.rb
+++ b/activerecord/test/cases/connection_adapters/merge_and_resolve_default_url_config_test.rb
@@ -46,6 +46,14 @@ module ActiveRecord
         assert_equal expected, actual
       end
 
+      def test_resolver_with_nil_database_url_and_current_env
+        ENV["RAILS_ENV"] = "foo"
+        config = { "foo" => { "adapter" => "postgres", "url" => ENV["DATABASE_URL"] } }
+        actual   = resolve_spec(:foo, config)
+        expected = { "adapter" => "postgres", "url" => nil, "name" => "foo" }
+        assert_equal expected, actual
+      end
+
       def test_resolver_with_database_uri_and_current_env_symbol_key_and_rack_env
         ENV["DATABASE_URL"] = "postgres://localhost/foo"
         ENV["RACK_ENV"]     = "foo"