core data - iOS: How to use predicates NSexpression to check user input for validity and conformance -

in app using coredata have entity 2 attributes gridvalue(numerical value), recordingdate(nsdate)). user in 1 tableview can enter these 2 values each day , recorded fetch controller. more understanding here screenshot of two views concerned issue:

th addviewcontroller user adds new value , when user clicks on of 2 fields in static tableview, redirected "zahl datum" view editing.

what check gridvalue entered user , display warning(could alert message or in case footer message show or pop whenever new entered value less previous value entered in different day(a previous stored value of recordingdate(the nsdate input attribute coredata model).

example illustrate if current user enters: zahlerstand = 1254 , datum: 12,05,2013, , in previous record values were: zahlerstand = 1300 , datum: 11.05.2013. see new value smaller old value on successive dates. in case warning should displayed(as in tableview section footer. before can click on "save" button in addviewcontroller.

please sample code if possible.

thank in advance,

use nscomparisonresult comparing older date newer date :

compare: method returns nscomparisonresult value indicates temporal ordering of receiver , given date.  - (nscomparisonresult)compare:(nsdate *)anotherdate  switch ([olderdate compare:newdate]) { case nsorderedascending:     // olderdate earlier in time newdate     break; case nsorderedsame:     // both dates same     break; case nsordereddescending:     // olderdate later in time newdate     break; }