I have added an extra field which relates to the project.project model in the sale order

class InheritSaleOrder(models.Model):
    _inherit = 'sale.order'

    project_name = fields.Many2one(comodel_name="project.project", string="Project Name", required=False)

i also have a customized module for project budget which is supposed  to automatically pick the all the products in the sale order line relating to a specific order using onchange here is my function

@api.multi
@api.onchange('project_id')
def change_project_id(self):
    #get quotation id
    quotation_id = self.project_id

    if(quotation_id!=""):

        #get quotation info
        quotation_list = self.env['sale.order'].search([('project_name','=',quotation_id.id)])
        data = {}
        #loop through quotation lines
        new_lines = self.env['project.budget.line']
        for line in quotation_list.order_line: #line 83
            ##populate items
            data = {
                'product_id': line.product_id.id,
                'product_categ_id':line.product_categ_id,
                'product_qty':line.product_uom_qty ,
                'product_uom' : line.product_uom.id ,
                'price_unit': line.product_id.standard_price #TODO - Use the pricelist to retrieve item prices
            }
            new_line = new_lines.new(data) #adding new items to the model
            new_lines += new_line
        #self.update('reference_bq' : value)
        self.budget_line_items += new_lines # set budget lines
        return {}
    #return vals

    # on change update lines

however when i try to run my code i get a singleton error i dont know what am missing kindly assist

File "/home/dishon/Desktop/odoo-dev/todo_app/project_budget/models/models.py", line 83, in change_project_id
    for line in quotation_list.order_line:
  File "/home/dishon/Desktop/odoo-dev/odoo/odoo/fields.py", line 909, in __get__
    record.ensure_one()
  File "/home/dishon/Desktop/odoo-dev/odoo/odoo/models.py", line 4848, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: sale.order(23, 22, 21)