Working as a Salesforce Developer meets with the various problems of both a configuration and developing.
One of the problems was when I created a Master-Details field on a custom object to another custom object. When I created a record on the child, I couldn’t edit the value of this field.
As it turned out this inner Salesforce limit (no edit option in the Master-Detail field regardless of the permissions to read / write). Fortunately, with the Summer 12 release we can change this value. To do this, enable the “Allow reparenting” in Master-Detail field definition. By default, you cannot “reparent” records in master-detail relationship.