Hi I am new to Apex and need some guidance.

Investors
Bonds_Pitched__c –> The total number of Bond_Buy__c that have been pitched (offered) to the investor.
Bonds_Purchased__c –> The total number of Bond_Buy__c the investor has purchased.

Bond Offerings
Each bond offering is for a total of
$1,000,000 and is composed of 1,000 units of $1,000.
Units_Pending__c –> The number of $1,000 Bond_Buy__c units that are being pitched to investors but have not yet been purchased or rejected.
Units_Sold__c –> The total number of $1,000 Bond_Buy__c units that have been purchased. 

Bond Buy
Bond_Offering__c –> The bond offering being purchased.
Investor__c –> The investor being pitched the bond offering.
Units__c –> The number of $1,000 bond units that are being offered.
Status__c –> The status of the bond buy. Valid values are Pending, Passed, and Purchased.


* Requirement
When changing any of these four custom fields (Bond_Offering__c(it's a lookup), Investor__c(it's a lookup), Status__c, Units__c) on custom object Bond_Buy__c, 2 custom fields (Bonds_Pitched__c and Bonds_Purchased__c) on custom object Investor__c and 2 custom fields (Units_Pending__c and Units_Sold__c) on custom object Bond_Offering__c need to be updated.
The updates should occur in real-time and should accommodate when Bond_Buy__c are created, edited, deleted, and undeleted.

Notice:roll-up summary NOT allowed (cause it is NOT master-detail relationship)

Below is my code. I am using trigger on Bond_Buy__c. It might be wrong.
*The scenario is I may have 0 in Bonds_Pitched__c or Bonds_Purchased__c or Units_Pending__c or Units_Sold__c fields when the Investors or Bond Offerings HAS one or more Bond Buy.
I guess the problem might be ----> else  {   inv.Bonds_Pitched__c =0;  } in the for loop.
But i have no idea how to fix it.
​Please help.
 

trigger BondBuyUpdate on Bond_Buy__c (after insert, after update) {
     
   //Limit the size of list by using Sets which do not contain duplicate elements
    set<Id> InvestorIds = new set<Id>();    
    set<Id> InvestorIdsP = new set<Id>();
    set<Id> BondOfferingIdsP = new set<Id>();
    set<Id> BondOfferingIdsS = new set<Id>();
    
  
    
    //When adding new Bond_Buy__c or updating existing Bond_Buy__c

        for(Bond_Buy__c bonbuy : trigger.new){
            if(Trigger.isInsert ||(Trigger.isUpdate && (Trigger.oldmap.get(bonbuy.id).Status__c!=bonbuy.Status__c || 
                                                        Trigger.oldmap.get(bonbuy.id).Units__c!=bonbuy.Units__c || 
                                                        Trigger.oldmap.get(bonbuy.id).Bond_Offering__c!= bonbuy.Bond_Offering__c|| 
                                                        Trigger.oldmap.get(bonbuy.id).Investor__c!=bonbuy.Investor__c)))
            {
                InvestorIds.add(bonbuy.Investor__c);
                
                InvestorIdsP.add(bonbuy.Investor__c);
                BondOfferingIdsP.add(bonbuy.Bond_Offering__c);
                BondOfferingIdsS.add(bonbuy.Bond_Offering__c);
            }
            
        }
    
    //Map will contain one Investor/BondOffering Id to one sum value
    map<Id,Double> InvestorIdsPitched  = new map <Id,Double>();
    map<Id,Double> InvestorIdsPurchased  = new map <Id,Double>();
    map<Id,Double> BondOfferingIdsPending  = new map <Id,Double>();
    map<Id,Double> BondOfferingIdsSold = new map <Id,Double>();
    
   
    
    //Produce a sum of units on child object and add them to the map
    //use group by to have a single parent Id with a single sum value
    
    
    
    //Investor Bonds_Pitched__c new update
        for(AggregateResult q : [select Investor__c,sum(Units__c)s1 from Bond_Buy__c where Investor__c IN :InvestorIds group by Investor__c]){
            InvestorIdsPitched.put((Id)q.get('Investor__c'),(Double)q.get('s1'));      
        }
        List<Investor__c> InvestorToUpdatePitched = [Select Id, Bonds_Pitched__c  from Investor__c where Id IN :InvestorIds];
        
        
        for(Investor__c inv : InvestorToUpdatePitched ){
             if(InvestorIdsPitched.containsKey(inv.Id))
            {                
                 inv.Bonds_Pitched__c = InvestorIdsPitched.get(inv.Id);
                
            }
            else
            {
                inv.Bonds_Pitched__c =0;         
            }
          
            system.debug('resultPitched '+inv.Bonds_Pitched__c);
        }
        
        if(InvestorToUpdatePitched.size()>0)
        {
            update InvestorToUpdatePitched;
        }
      
    
   
    
    //Investor Bonds_Purchased__c new update
    
        for(AggregateResult q : [select Investor__c,sum(Units__c)s2 from Bond_Buy__c where Status__c='Purchased' and Investor__c IN :InvestorIdsP  group by Investor__c]){
            InvestorIdsPurchased.put((Id)q.get('Investor__c'),(Double)q.get('s2'));
        }
        
        List<Investor__c> InvestorToUpdatePurchased = [Select Id, Bonds_Purchased__c  from Investor__c where Id IN :InvestorIdsP];
        
        for(Investor__c inv2 : InvestorToUpdatePurchased ){
            if(InvestorIdsPurchased.containsKey(inv2.Id))
            {
                
                 inv2.Bonds_Purchased__c = InvestorIdsPurchased.get(inv2.Id); 
                
            }
            else
            {
                inv2.Bonds_Purchased__c =0;
            }
            
            system.debug('resultPurchased '+inv2.Bonds_Purchased__c);
        }
        if(InvestorToUpdatePurchased.size()>0)
        {
            update InvestorToUpdatePurchased;
        }
        
    
   
    
   //Bond_Offering__c Units_Pending__c new update
        for(AggregateResult q : [select Bond_Offering__c,sum(Units__c)s3 from Bond_Buy__c where Status__c='Pending' and Bond_Offering__c IN :BondOfferingIdsP  group by Bond_Offering__c]){
            BondOfferingIdsPending.put((Id)q.get('Bond_Offering__c'),(Double)q.get('s3'));
        }
        
        List<Bond_Offering__c> BondOfferingToUpdatePending = [Select Id, Units_Pending__c from Bond_Offering__c where Id IN :BondOfferingIdsP];
        
        
        for(Bond_Offering__c bon : BondOfferingToUpdatePending){
            
            if(BondOfferingIdsPending.containsKey(bon.Id))
            {
                bon.Units_Pending__c = BondOfferingIdsPending.get(bon.Id);  
            }
            else
            {
                bon.Units_Pending__c =0;
            }
            
        }
        if(BondOfferingToUpdatePending.size()>0)
        {
            update BondOfferingToUpdatePending;
        }
    
    
    
    
    //Bond_Offering__c Units_Sold__c new update
        for(AggregateResult q : [select Bond_Offering__c,sum(Units__c)s4 from Bond_Buy__c where Status__c='Purchased' and Bond_Offering__c IN :BondOfferingIdsS  group by Bond_Offering__c]){
            BondOfferingIdsSold.put((Id)q.get('Bond_Offering__c'),(Double)q.get('s4'));
        }
        
        
        
        List<Bond_Offering__c> BondOfferingToUpdateSold = [Select Id, Units_Sold__c from Bond_Offering__c where Id IN :BondOfferingIdsS];
        
        for(Bond_Offering__c bon2 : BondOfferingToUpdateSold){
            
            if(BondOfferingIdsSold.containsKey(bon2.Id))
            {
                bon2.Units_Sold__c = BondOfferingIdsSold.get(bon2.Id);
            }
            else
            {
                bon2.Units_Sold__c =0;
            }
            
        }
        if(BondOfferingToUpdateSold.size()>0)
        {
            update BondOfferingToUpdateSold;
        } 
    
    
    
  
}

trigger BondBuyUpdate2 on Bond_Buy__c (after update, after delete, after undelete) {

    set<Id> InvestorIdsOlder = new set<Id>();
    set<Id> InvestorIdsPOlder = new set<Id>();
    set<Id> BondOfferingIdsPOlder = new set<Id>();
    set<Id> BondOfferingIdsSOlder = new set<Id>();
    
     for(Bond_Buy__c bonbuy : trigger.old)
     {
      
         {
                InvestorIdsOlder.add(bonbuy.Investor__c);               
                InvestorIdsPOlder.add(bonbuy.Investor__c);
                BondOfferingIdsPOlder.add(bonbuy.Bond_Offering__c);
                BondOfferingIdsSOlder.add(bonbuy.Bond_Offering__c);
            }
     }
    
     map<Id,Double> InvestorIdsPitchedOld  = new map <Id,Double>();
    map<Id,Double> InvestorIdsPurchasedOld  = new map <Id,Double>();
    map<Id,Double> BondOfferingIdsPendingOld  = new map <Id,Double>();
    map<Id,Double> BondOfferingIdsSoldOld = new map <Id,Double>();
    
     //Investor Bonds_Pitched__c old update 
    List<Investor__c> InvestorToUpdatePitchedOld = [Select Id, Bonds_Pitched__c  from Investor__c where Id IN :InvestorIdsOlder];
    for(Investor__c inv : InvestorToUpdatePitchedOld ){
        
                 if(InvestorIdsPitchedOld.containsKey(inv.Id))
            {
                
                 inv.Bonds_Pitched__c = InvestorIdsPitchedOld.get(inv.Id); 
                
            }
            else
            {
                inv.Bonds_Pitched__c =0;
            }
           
          
            system.debug('resultPitched '+inv.Bonds_Pitched__c);
        }  
    
    if(InvestorToUpdatePitchedOld.size()>0)
        {
            update InvestorToUpdatePitchedOld;
        }
    
     //Investor Bonds_Purchased__c old update
    
    List<Investor__c> InvestorToUpdatePurchasedOld = [Select Id, Bonds_Purchased__c  from Investor__c where Id IN :InvestorIdsPOlder];
    
    for(Investor__c inv2 : InvestorToUpdatePurchasedOld ){
            if(InvestorIdsPurchasedOld.containsKey(inv2.Id))
            {
                
                 inv2.Bonds_Purchased__c = InvestorIdsPurchasedOld.get(inv2.Id); 
                
            }
            else
            {
                inv2.Bonds_Purchased__c =0;
            }
            
            system.debug('resultPurchased '+inv2.Bonds_Purchased__c);
        }
        if(InvestorToUpdatePurchasedOld.size()>0)
        {
            update InvestorToUpdatePurchasedOld;
        }
    
    //Bond_Offering__c Units_Pending__c old update
    List<Bond_Offering__c> BondOfferingToUpdatePendingOld = [Select Id, Units_Pending__c from Bond_Offering__c where Id IN :BondOfferingIdsPOlder];
    
     for(Bond_Offering__c bon : BondOfferingToUpdatePendingOld){
            
            if(BondOfferingIdsPendingOld.containsKey(bon.Id))
            {
                bon.Units_Pending__c = BondOfferingIdsPendingOld.get(bon.Id);  
            }
            else
            {
                bon.Units_Pending__c =0;
            }
            
        }
        if(BondOfferingToUpdatePendingOld.size()>0)
        {
            update BondOfferingToUpdatePendingOld;
        }
    
    //Bond_Offering__c Units_Sold__c old update
    
    List<Bond_Offering__c> BondOfferingToUpdateSoldOld = [Select Id, Units_Sold__c from Bond_Offering__c where Id IN :BondOfferingIdsSOlder];
    
    for(Bond_Offering__c bon2 : BondOfferingToUpdateSoldOld){
            
            if(BondOfferingIdsSoldOld.containsKey(bon2.Id))
            {
                bon2.Units_Sold__c = BondOfferingIdsSoldOld.get(bon2.Id);
            }
            else
            {
                bon2.Units_Sold__c =0;
            }
            
        }
        if(BondOfferingToUpdateSoldOld.size()>0)
        {
            update BondOfferingToUpdateSoldOld;
        } 
}