allow to access Document via the db

gistfile1.py

class MyDoc(Document):
     collection_name = "mydoc"
     structure = {
          'foo': unicode,
          'bar': int,
     }

con = Connection()
con.register([MyDoc])

db = con.mydb

# this will be allowed and will use the `collection_name` attribute
doc = db.MyDoc()

# but this is also supported
doc = db.mycol.MyDoc()

# if the collection is different with the value of `collection_name`, a warning is raised in logging:
doc = db.othercol.MyDoc()
# Warning : the collection "othercol" is used instead of "mydoc"