Corrected custom model fields how-to.
Corrected custom model fields how-to.

get_prep_value() method is complementary of from_db_value().

Follow up to e9103402c0fa873aea58a6a11dba510cd308cb84.

takis/django,shaib/django,felixxm/django,django/django,freakboy3742/django,googleinterns/django,adamchainz/django,auvipy/django,dsanders11/django,dsanders11/django,felixxm/django,elena/django,MarkusH/django,pauloxnet/django,monetate/django,koordinates/django,felixxm/django,apollo13/django,blighj/django,rsalmaso/django,ghickman/django,dsanders11/django,auvipy/django,freakboy3742/django,monetate/django,django/django,django/django,rsalmaso/django,manhhomienbienthuy/django,shaib/django,atul-bhouraskar/django,pauloxnet/django,ghickman/django,shaib/django,rsalmaso/django,takis/django,MarkusH/django,solarissmoke/django,adamchainz/django,atul-bhouraskar/django,manhhomienbienthuy/django,manhhomienbienthuy/django,freakboy3742/django,django/django,atul-bhouraskar/django,googleinterns/django,atul-bhouraskar/django,blighj/django,monetate/django,elena/django,blighj/django,solarissmoke/django,wkschwartz/django,koordinates/django,solarissmoke/django,adamchainz/django,koordinates/django,elena/django,wkschwartz/django,MarkusH/django,blighj/django,auvipy/django,dsanders11/django,wkschwartz/django,adamchainz/django,koordinates/django,ghickman/django,MarkusH/django,shaib/django,takis/django,takis/django,apollo13/django,googleinterns/django,solarissmoke/django,pauloxnet/django,apollo13/django,manhhomienbienthuy/django,ghickman/django,pauloxnet/django,rsalmaso/django,elena/django,apollo13/django,monetate/django,googleinterns/django,wkschwartz/django,felixxm/django,auvipy/django