The better way to go about this would be to use self.env to perform the search. The code you have executes an XMLRPC event. Using self.env, your code would look like

result = self.env['mrp.workorder'].search([('production_id', '=', '3')])

not sure, this is best answer, but atlease i managed to solve this, by adding line below in python function :)

for xml rpc (external client)

result = models.execute_kw(db, uid, password, 'mrp.workorder', 'search', [[['production_id
    ...: ', '=', 3]]])

For Module Code(using self.env)

• simple search records

relWorkOrders = self.env['mrp.workorder'].search([('production_id','=',self.id)])

• search and read specific fields in one go (i ended up using this)
  

mo = self.env['mrp.workorder']

wo_fields = ['name','state','workcenter_id']

relWorkOrdersData = mo.search_read([('production_id','=',self.id)],wo_fields)