I'm using odoo 8 

Hi I am trying to use the portal to allow customers to view and create incidents for a project. 
When I project_issues permissions for the object and assign read and write permissions, this works well, the complete and save the Project field displays an error. 
When this field contains any data the incidence gives me a permissions error. 
The'm creating new, should be sufficient permission to create. 
If I assign write permission, this works well. 
  Any idea to solve this? 
I'm not using rules. 

thanks