If you want to use classical inheritance in OpenERP use below code -

class subscription_document(osv.osv):
    _name = "subscription.document"
    _description = "Subscription Document"
    _columns = {
        'name': fields.char('Name', size=60, required=True),
        'active': fields.boolean('Active', help="If the active field is set to False, it will allow you to hide the subscription document without removing it."),
        'model': fields.many2one('ir.model', 'Object', required=True),
        'field_ids': fields.one2many('subscription.document.fields', 'document_id', 'Fields')
    }
    _defaults = {
        'active' : lambda *a: True,
    }
subscription_document()

class subscription_document_fields(osv.osv):
    _name = "subscription.document.fields"
    _description = "Subscription Document Fields"
    _rec_name = 'field'
    _columns = {
        'field': fields.many2one('ir.model.fields', 'Field', domain="[('model_id', '=', parent.model)]", required=True),
        'value': fields.selection([('false','False'),('date','Current Date')], 'Default Value', size=40, help="Default value is considered for field when new document is generated."),
        'document_id': fields.many2one('subscription.document', 'Subscription Document', ondelete='cascade'),
    }
    _defaults = {}
subscription_document_fields()