Rails 6: add database_exists? (#796)

* add database_exists?
rails/rails#36471

* fix translate error to ActiveRecord::NoDatabaseError in connection creating
add "test bad connection" like other adapters

lib/active_record/connection_adapters/sqlserver_adapter.rb

@@ -81,6 +81,12 @@ def schema_creation
         SQLServer::SchemaCreation.new self
       end
 
+      def self.database_exists?(config)
+        !!ActiveRecord::Base.sqlserver_connection(config)
+      rescue ActiveRecord::NoDatabaseError
+        false
+      end
+
       def supports_ddl_transactions?
         true
       end

lib/active_record/sqlserver_base.rb

@@ -11,6 +11,12 @@ def sqlserver_connection(config) #:nodoc:
         raise ArgumentError, "Unknown connection mode in #{config.inspect}."
       end
       ConnectionAdapters::SQLServerAdapter.new(nil, nil, config.merge(mode: mode))
+    rescue TinyTds::Error => e
+      if e.message.match(/database .* does not exist/i)
+        raise ActiveRecord::NoDatabaseError
+      else
+        raise
+      end
     end
   end
 end

test/cases/adapter_test_sqlserver.rb

@@ -65,6 +65,25 @@ class AdapterTestSQLServer < ActiveRecord::TestCase
     assert_equal 'customers', connection.send(:get_table_name, basic_select_sql)
   end
 
+  it 'test bad connection' do
+    assert_raise ActiveRecord::NoDatabaseError do
+      config = ActiveRecord::Base.configurations['arunit'].merge(database: 'inexistent_activerecord_unittest')
+      ActiveRecord::Base.sqlserver_connection config
+    end
+  end
+
+  it 'test database exists returns false if database does not exist' do
+    config = ActiveRecord::Base.configurations['arunit'].merge(database: 'inexistent_activerecord_unittest')
+    assert_not ActiveRecord::ConnectionAdapters::SQLServerAdapter.database_exists?(config),
+                'expected database to not exist'
+  end
+
+  it 'test database exists returns true when the database exists' do
+    config = ActiveRecord::Base.configurations['arunit']
+    assert ActiveRecord::ConnectionAdapters::SQLServerAdapter.database_exists?(config),
+            "expected database #{config[:database]} to exist"
+  end
+
   describe 'with different language' do
 
     before do