I have a custom report , with a print button(red) , however when i click the print button i get this error :

           File "/opt/odoo/community/addons/camden-market/cm_sales_receipt/cm_sales_receipt.py", line 89, in print_receipt assert len(self) == 1, 'This option should only be used for a single id at a time.' File "/opt/odoo/odoo/openerp/models.py", line 5332, in __len__ return len(self._ids) AttributeError: 'cm.sales.receipt' object has no attribute '_ids'

Also i have another normal print button which when i click print my report but it is just an empty file.

My class :

class cm_sales_receipt(osv.osv):
    _name = "cm.sales.receipt"
    _description = "Camden Market Sales Receipt"
    _auto = False
    _columns = {
        'paymentref':fields.char('PaymentRef'),
        'date':fields.date('Date'),
        'receipt_number':fields.char('Receipt Number'),
        'customer':fields.char('Customer',size=128),
        'unit':fields.char('Unit',size=128),
        'company':fields.char('Name',size=128,required=True),
        'invoicing_company':fields.char('Invoicing Company', size=128,required=True),
        'street':fields.char('Street',size=128,required=True),
        'street2':fields.char('Street2',size=128,required=True),
        'zip':fields.char('Zip',size=10,required=True),
        'state':fields.char('State',size=128,required=True),
        'city':fields.char('City',size=128,required=True),
        'email':fields.char('Email',size=128),
        'fax':fields.integer('Fax'),
        'phone':fields.integer('State'),
        'paid_amount':fields.float('Paid_Amount'),

    }

Here below is the code used  to print the report (note function is indented so part of above class :

 def print_receipt(self, cr, uid, ids, context=None):
        """
        This function prints the Property Rental Sales receipt
        """
        assert len(self) == 1, 'This option should only be used for a single id at a time.'
        #self.sent = True
        return self.pool['report'].get_action(cr, uid, ids, 'cm_sales_receipt.report_salesreceipt', context=context)

extract from view which has print button (note: print button displays correctly in module:

 <!-- Property Rental Sales Receipt Form View -->
        <record model="ir.ui.view" id="view_cm_sales_receipt_form">
            <field name="name">cm.sales.receipt.form</field>
            <field name="model">cm.sales.receipt</field>
            <field name="arch" type="xml">
                <form string=" Sales Receipt" version="8.0">
                <header>
                    <button name="print_receipt" string="Print Receipt" type="object" class="oe_highlight" groups="base.group_user"/>
                </header>
                                <sheet>
                                <group>
                                        <group>
                                               <field name="invoicing_company"/>
                           <field name="customer"/>
                                               <field name="paymentref"/>
                           <field name="receipt_number"/>
                           <field name="company"/>
                                               <field name="paid_amount"/>
                                    </group>
                                        </group>
                                        </sheet>
                </form>
            </field>
        </record>

here is the report xml file :

<openerp>
    <data>
        <report
            string="Sales Rental Receipt"
            id="action_cm_report_sales_receipt"
            model="cm.sales.receipt"
            report_type="qweb-pdf"
            name="cm_sales_receipt.report_cm_salesreceipt"
            file="cm_sales_receipt.report_cm_salesreceipt"
            attachment_use="True"
            attachment="(object.state in ('open','paid')) and ('RCPT'+(object.number or '').replace('/','')+'.pdf')"
        />
    </data>
</openerp

• module installs correctly
• ((custom)print button displays correctly but once clicked i recieve an  error
• when i click the normal print button once clicked it prints  an empty report.

1) it seems the _ids cannot be found in this case , is there an indication of where in the code is '_ids' is not being passed through .