Hi All, I have problem with copy one2many field. I create assignment field for Project:

assignment_ids = fields.One2many(
        string='Project Assignments',
        comodel_name='project.assignment',
        inverse_name='project_id',
        track_visibility='onchange',)

First I try to add attribute copy=True and then it throws an error 

Expected singleton: project.assignment(3, 4) 

and this has the same error singleton:

    @api.multi
    def copy(self, default=None):
        default = default or {}
        list_assignment = self.assignment_ids
        new_assignment = []
        for assignment in list_assignment:
            new_assignment.append((0, 0, {'id': assignment.id,
                                        'role_id': assignment.role_id.id,
                                        'date_join': date.today(),
                                        'user_id': assignment.user_id.id,
                                        }))
        default['assignment_ids'] = new_assignment

        return super(ProjectProject, self).copy(default)

Then I try to override the copy function. The assignment have a copy to new Project but old Project's assignment has been deleted.

 @api.multi    
def copy(self, default=None):
        default = default or {}
        list_assignment = self.assignment_ids

        default['assignment_ids'] = (6, True, list_assignment)

        return super(ProjectProject, self).copy(default)

So how can I copy this one2many field?

Thank you so much!!!