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)