I have a situation that whenever I tried to open the wizard i want my data back in the one2many fields since for the parent is working fine it is loading the data by using _default as a prefix over every fields but what if i want to load the data in the grid(One2many) also

# Here is the Main class

class batch_wise_allocation(models.TransientModel):
    _name = 'batch.wise_allocation'
    _table = 'batch_wise_allocation'
    _description = 'Batch Wise Allocation'
  
    product_id= fields.Many2one('product.product', string="Product")
    dispatch_line_id = fields.Integer(string= 'Dispatch Line ID')
    main_id=  fields.One2many('batch.wise_allocation_line','batch_allocation_line_id',string= 'Allocation Line ID')

#Here is the child class

class batch_wise_allocation_line(models.TransientModel):
    _name = 'batch.wise_allocation_line'
    _table = 'batch_wise_allocation_line'
    _description = 'Batch Wise Allocation Line'
  
    batch_allocation_line_id= fields.Many2one('batch.wise_allocation', string="Batch Allocation ID")
    batch_id= fields.Many2one('pra.batch_master',string= 'Batch No')
    batch_name=fields.Char('Batch Name')
    batch_size= fields.Float('Batch Size')
    batch_qty=fields.Float('Batch Qty')
    dispatch_qty= fields.Float('Dispatch Qty')
    packing_details= fields.Char('Packing Details')


#########

And this is the class from where I want to load the batch_list and the batch_wise_dispatch automatically if it is entered before while opening the wizard

class DispatchLine(models.Model):
    _name = 'pra.dispatch_line'
    _table = 'pra_dispatch_line'
    _description = 'Sales Dispatch Line'
  
    def action_dispatch_qty_transfer(self, cr, uid, ids, context=None):
        pra_dispatch_line = self.browse(cr, uid, ids[0], context=context)
        return {
            'name': ('Batch Allocation'),
            'view_type':'form',
            'view_mode':'form',
            'res_model': 'batch.wise_allocation',
            'view_id':False,
            'type':'ir.actions.act_window',
            'target':'new',
            'context': {'default_dispatch_line_id':pra_dispatch_line.id,
                        'default_product_id':pra_dispatch_line.product_id.id,

                         #How can I add here the default for the One2many fields------????
                        },
            }
  
  
    batch_list = fields.Text(string= 'Batches',readonly=1)
    batch_wise_dispatch = fields.Text(string= 'Batch Wise Dispatch',readonly=1)

#############################################
'''Very thanks to Dan Čermák for the last 2 answer it was really very wonderful'''