diff --git a/activerecord/lib/active_record/database_configurations.rb b/activerecord/lib/active_record/database_configurations.rb
index 44b5cfc738..b917f4e6b7 100644
--- a/activerecord/lib/active_record/database_configurations.rb
+++ b/activerecord/lib/active_record/database_configurations.rb
@@ -141,7 +141,7 @@ module ActiveRecord
           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 })
+        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|
diff --git a/activerecord/test/cases/connection_adapters/connection_handler_test.rb b/activerecord/test/cases/connection_adapters/connection_handler_test.rb
index 27589966af..843242a897 100644
--- a/activerecord/test/cases/connection_adapters/connection_handler_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handler_test.rb
@@ -29,7 +29,7 @@ module ActiveRecord
 
       def test_establish_connection_uses_spec_name
         old_config = ActiveRecord::Base.configurations
-        config = { "readonly" => { "adapter" => "sqlite3" } }
+        config = { "readonly" => { "adapter" => "sqlite3", "pool" => "5" } }
         ActiveRecord::Base.configurations = config
         resolver = ConnectionAdapters::ConnectionSpecification::Resolver.new(ActiveRecord::Base.configurations)
         spec =   resolver.spec(:readonly)
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 515bf5df06..c0a9f8f9ca 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
@@ -273,6 +273,37 @@ module ActiveRecord
                     }
         assert_equal expected, actual
       end
+
+      def test_merge_no_conflicts_with_database_url_and_adapter
+        ENV["DATABASE_URL"] = "postgres://localhost/foo"
+
+        config   = { "default_env" => { "adapter" => "postgresql", "pool" => "5" } }
+        actual   = resolve_config(config)
+        expected = { "default_env" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost",
+                       "pool"     => "5"
+                     }
+        }
+        assert_equal expected, actual
+      end
+
+      def test_merge_no_conflicts_with_database_url_and_numeric_pool
+        ENV["DATABASE_URL"] = "postgres://localhost/foo"
+
+        config   = { "default_env" => { "pool" => 5 } }
+        actual   = resolve_config(config)
+        expected = { "default_env" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost",
+                       "pool"     => 5
+                     }
+        }
+
+        assert_equal expected, actual
+      end
     end
   end
 end