Trigger Calculation between two fields from different sObjects

Question

I am new to apex. I have two objects blood_bank__c and patient__c. I want to change quantity of blood in blood bank after patient is given some blood. This trigger is not working, and its not allowing me to add new patient.

trigger :

trigger bloodcalTrigger on patient__c ( after update, after insert) {

patient__c tok = [ Select blood_quantity_used__c, Blood_group_del__c From patient__c];
blood_bank__c cons = [Select quantity__c, name From blood_bank__c];


          if(tok.Blood_group_del__c == cons.name )  {
   cons.quantity__c = cons.quantity__c - tok.blood_quantity_used__c ;

   }else {}

}

Answer 1

You are not having any DML statement that your blood_bank__c record is updated.

Try

cons.quantity__c = cons.quantity__c - tok.blood_quantity_used__c ; update cons;

However, you assume that you only have one blood bank in the system. You should try to also update

blood_bank__c cons = [Select quantity__c, name From blood_bank__c];

to something like

blood_bank__c cons = [Select quantity__c, name From blood_bank__c WHERE name = :tok.Blood_group_del__c];

If you need more help, we need additional information about your data structure.