i just wrote this code:

trigger updateQuant on Order__c (after insert) {


    Map <id,Item__c> ma=new Map<id,Item__c>();
    for(Order__c o:Trigger.New){
        ma.put(o.Item__c,null);
        
    for(Item__c i: [select id,Price__c,Quantity__c from Item__c where 
                                                id in:ma.keySet()])
        ma.put(i.id,i);
               
               
    for(Order__c q:Trigger.New){
        Item__c t= ma.get(q.Item__c);                                                 
        t.Quantity__c=t.Quantity__c-1;   
    }    
}
}

i tried to chage the Quantity number in the parent  item but it didn't change, where is my mistake?
thanks for the help

i just wrote this code:

trigger updateQuant on Order__c (after insert) {


    Map <id,Item__c> ma=new Map<id,Item__c>();
    for(Order__c o:Trigger.New){
        ma.put(o.Item__c,null);
        
    for(Item__c i: [select id,Price__c,Quantity__c from Item__c where 
                                                id in:ma.keySet()])
        ma.put(i.id,i);
               
               
    for(Order__c q:Trigger.New){
        Item__c t= ma.get(q.Item__c);                                                 
        t.Quantity__c=t.Quantity__c-1;   
    }    
}
}

i tried to chage the Quantity number in the parent  item but it didn't change, where is my mistake?
thanks for the help