entc/gen: add open func for creating client

Summary: Pull Request resolved: https://github.com/facebookincubator/ent/pull/44

Reviewed By: alexsn

Differential Revision: D17657719

fbshipit-source-id: 9bbe18dc0e9c553f8a2df81018d7db06f38f5e0e

examples/m2m2types/ent/client.go

@@ -43,6 +43,23 @@ func NewClient(opts ...Option) *Client {
 	}
 }
 
+// Open opens a connection to the database specified by the driver name and a
+// driver-specific data source name, and returns a new client attached to it.
+// Optional parameters can be added for configuring the client.
+func Open(driverName, dataSourceName string, options ...Option) (*Client, error) {
+	switch driverName {
+	case dialect.MySQL, dialect.SQLite:
+		drv, err := sql.Open(driverName, dataSourceName)
+		if err != nil {
+			return nil, err
+		}
+		return NewClient(append(options, Driver(drv))...), nil
+
+	default:
+		return nil, fmt.Errorf("unsupported driver: %q", driverName)
+	}
+}
+
 // Tx returns a new transactional client.
 func (c *Client) Tx(ctx context.Context) (*Tx, error) {
 	if _, ok := c.driver.(*txDriver); ok {
@@ -80,6 +97,11 @@ func (c *Client) Debug() *Client {
 	}
 }
 
+// Close closes the database connection and prevents new queries from starting.
+func (c *Client) Close() error {
+	return c.driver.Close()
+}
+
 // GroupClient is a client for the Group schema.
 type GroupClient struct {
 	config