How to copy one2many filed in odoo?

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

Error while validating constraint

Expected singleton: project.assignment(3, 4)

And this has the same singleton error

@api.multi
def copy(self, default=None):
    default = default or {}
    for rec in self:
        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!!!

This code works when I copied one2many from sale to one2many field in invoice.

def _prepare_invoice(self):
    self.ensure_one()
        res=return super([Your_Class_Name], self)._prepare_invoice()
# Loop in your One2many field in sale order
         one2many_vals =[]
          for l in self.order_id.[Your_One2many]:
              one2many_vals.append(
    (0,0,{'One2many_column1_in_invoice_One2Many':l.[One2many_column1_in_sale_One2Many],
          'One2many_column2_in_invoice_One2Many':l.[One2many_column2_in_sale_One2Many],
          'One2many_column3_in_invoice_One2Many':l.[One2many_column3_in_sale_One2Many]
    }))
res['One2many_field_name_in_invoice'] = one2many_vals
return res