I have a billing module that has a model "hospital.billing" with one2many relationship "hospital.account.lines" , it is like POS module with "pos.order" and pos.order.line",stock picking in POS is in pos.order model but my stock picking is in hospital account lines model since my transaction always in hospital account lines. Everytime the patient order a medicine  will be added its to Statement of Account. Since patient has only one SOA. This is my model stucture.
          
   _name="hospital.billing"
    _description="Hospital Billing System"
    
    _columns={
              
              'name' : fields.char ('Bill Number',required=True,states={'process':[('readonly',True)], 'cancel':[('readonly',True)]}, size=20),
              'date_processed': fields.date('Date Processed', select=True, readonly=True),
              'patient_case_id' : fields.many2one ('his.admission', 'Patient Admission', required=True, states={'process':[('readonly',True)], 'cancel':[('readonly',True)]}),
              'inpac_rmbd_invoices' : fields.one2many ('hospital.roomboard_invoice', 'name', 'In-Patient Room and Board Lines' ,states={'process':[('readonly',True)], 'cancel':[('readonly',True)]}),

              'billing_soa_lines' : fields.one2many ('hospital.account.lines', 'name', 'In-Patient SOA Lines' ,states={'process':[('readonly',True)], 'cancel':[('readonly',True)]}),
              'creditor_id' : fields.one2many ('creditor.mgmt', 'creditor', 'Creditors' ,states={'process':[('readonly',True)], 'cancel':[('readonly',True)]},domain=[('refund','=',False)]),
              'note': fields.text('Note' ,states={'process':[('readonly',True)], 'cancel':[('readonly',True)]}),
              'opd': fields.boolean('Out Patient'),
              'date_created':fields.datetime('Date Created'),
              'data_encoder':fields.many2one('res.users','Data Created by',readonly=True),
              'company_id':fields.related('data_encoder','company_id',type="many2one",relation="res.company",store=True,readonly=True,string="Company")

              
        _name="hospital.account.lines"
        _description ="Hospital Account lines"
        
        
        _columns={
                   'product_id':fields.many2one('product.product','Products',required=True),
                   'quantity':fields.float('Quantity',required=True),
                   'price_unit':fields.float('Price Unit',),
                   'discount':fields.float('Discount %'),
                   'price_subtotal':fields.function(price_subtotal,method=True,type="float",string="Price Subtotal w/o   Tax",store=False,readonly=True),
                   'creditor_amt':fields.function(creditor_amt,string='Creditor',type="float",method=True,readonly=True),
                   #'creditor_amt':fields.float('Creditor Amount',readonly=True),
                   'patient_amount':fields.function(patient_amount,method=True,type="float",string="Patient               Amount",store=False,readonly=True),
                   **'picking_id':fields.many2one('stock.picking','Stock Picking'),**
                   'pricelist_id':fields.many2one('product.pricelist','Product Pricelist'),
                   'primary_shop_id':fields.many2one('sale.shop','Primary Shop',),
                   'secondary_shop_id':fields.many2one('sale.shop','Secondary Shop'),
                   'date_created':fields.date('Date Created'),
                   'user_id':fields.many2one('res.users','Salesman'),
    }