I created a method that posts a message on every save to products.product. I post the message with:

self.message_post(cr, uid, pprecord['id'], body='My nice message body', subject='My nice message subject')

But now i would like to use an existing E-Mail Template instead of simple strings to post the message.

Is there already a simple method to directly post an E-Mail Template? 

I played around with: self.pool.get('email.template').render_template(cr, uid, 3, 'product.product', pprecord['id'], context=context) but all i want is to post the E-Mail template and not to read all the fields one by one...