Fix multiple database support for DATABASE_URL env variable

This commit fixes an issue where multi-database configurations were
incompatible with setting a `DATABASE_URL` environment variable.

As part of this work, this commit also includes a light refactor
to make both multi and single database configurations lead into the same
code path so they behave the same.

As mentioned in #36736, this regression was introduced as part of
f2ad69fe7a605b01bb7c37eeac6a9b4e7deb488e

2 files changed, 42 insertions, 12 deletions

diff --git a/activerecord/lib/active_record/database_configurations.rb b/activerecord/lib/active_record/database_configurations.rb
index bf31bb7c22..e122628b05 100644
--- a/activerecord/lib/active_record/database_configurations.rb
+++ b/activerecord/lib/active_record/database_configurations.rb
@@ -104,18 +104,28 @@ module ActiveRecord
         return configs.configurations if configs.is_a?(DatabaseConfigurations)
         return configs if configs.is_a?(Array)
 
-        build_db_config = configs.each_pair.flat_map do |env_name, config|
-          walk_configs(env_name.to_s, "primary", config)
-        end.flatten.compact
+        db_configs = configs.flat_map do |env_name, config|
+          if config.is_a?(Hash) && config.all? { |k, v| v.is_a?(Hash) }
+            walk_configs(env_name.to_s, config)
+          else
+            build_db_config_from_raw_config(env_name.to_s, "primary", config)
+          end
+        end.compact
 
         if url = ENV["DATABASE_URL"]
-          build_url_config(url, build_db_config)
+          merge_url_with_configs(url, db_configs)
         else
-          build_db_config
+          db_configs
+        end
+      end
+
+      def walk_configs(env_name, config)
+        config.map do |spec_name, sub_config|
+          build_db_config_from_raw_config(env_name, spec_name.to_s, sub_config)
         end
       end
 
-      def walk_configs(env_name, spec_name, config)
+      def build_db_config_from_raw_config(env_name, spec_name, config)
         case config
         when String
           build_db_config_from_string(env_name, spec_name, config)
@@ -141,16 +151,12 @@ module ActiveRecord
           config_without_url.delete "url"
 
           ActiveRecord::DatabaseConfigurations::UrlConfig.new(env_name, spec_name, url, config_without_url)
-        elsif config["database"] || config["adapter"] || ENV["DATABASE_URL"]
-          ActiveRecord::DatabaseConfigurations::HashConfig.new(env_name, spec_name, config)
         else
-          config.each_pair.map do |sub_spec_name, sub_config|
-            walk_configs(env_name, sub_spec_name, sub_config)
-          end
+          ActiveRecord::DatabaseConfigurations::HashConfig.new(env_name, spec_name, config)
         end
       end
 
-      def build_url_config(url, configs)
+      def merge_url_with_configs(url, configs)
         env = ActiveRecord::ConnectionHandling::DEFAULT_ENV.call.to_s
 
         if configs.find(&:for_current_env?)
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 6372abbf3f..15a045ed23 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
@@ -323,6 +323,30 @@ module ActiveRecord
 
         assert_equal expected, actual
       end
+
+      def test_tiered_configs_with_database_url
+        ENV["DATABASE_URL"] = "postgres://localhost/foo"
+
+        config = {
+          "default_env" => {
+            "primary" => { "pool" => 5 },
+            "animals" => { "pool" => 5 }
+          }
+        }
+
+        expected = {
+          "adapter"  => "postgresql",
+          "database" => "foo",
+          "host"     => "localhost",
+          "pool"     => 5
+        }
+
+        ["primary", "animals"].each do |spec_name|
+          configs = ActiveRecord::DatabaseConfigurations.new(config)
+          actual = configs.configs_for(env_name: "default_env", spec_name: spec_name).config
+          assert_equal expected, actual
+        end
+      end
     end
   end
 end