Trigger update of relations fields between objects

Question

i have problem with a exercise Writing a trigger in Apex that updates the field "location" of the Boat, when the "boat location" field of the associated Contact is modified, for all the boats that are linked to that Contact. The "Location of boat object" field should be the same as the "location of object contact boat

Boat = custom object-------location = (data type : picklist = Argentina,Brasil,España,India)------ field name= Contact, data type=Lookup Relationship

Contact = object standard----- boat_location = (data type : picklist = Argentina,Brasil,España,India)

my code is wrong and I don't know how to fix it

trigger UpdateLocation on Contact (before update) {

    if (Trigger.isUpdate) {
        List<Contact> cList = new List<Contaact>();

        Map<Id,Contact> acctsWithOpps = new Map<Id,Contact>(
        [SELECT Id,(SELECT Id FROM Boat) FROM Contact WHERE Id IN :Trigger.New]);

}
}

Answer 1

Generally, when you want to update another object in trigger, you would do it in After event trigger.

trigger UpdateLocation on Contact (after update) {
    if (trigger.isAfter && Trigger.isUpdate) {
        List<Boat__c> boats = new List<Boat__c>();
        for (Contact c : [
                    SELECT location_boat__c, (SELECT Id FROM Boats__r)
                    FROM Contact
                    WHERE Id = :trigger.new
                ]) {
            for (Boat__c b : c.Boats__r) {
                b.location__c = c.location_boat__c;
                boats.add(b);
            }
        }
        update(boats);
    }
}

The relationship name of Boats__r in the code above is just my assumption. You can find the exact name from the Custom Field Definition Detail page of the Boat's Location field.

Answer 2

Yes your code doesn't make sense at all. There is data type mismatch (you can't assign Opportunity list to Contact list, is that a typo?). And you are not doing anything other than just quering contact records.

Try this (handle bulk updates too and each line is easy to understand, you can seach the few keywords which is used below to understand in detail)

trigger UpdateLocation on Contact (before update) {
    Map<Id, Contact> updatedLocationContacts = new Map<Id, Contact>();
    for (Contact c : Trigger.new) {
        if (c.location_boat__c != Trigger.oldMap.get(c.Id).location_boat__c) {
            updatedLocationContacts.put(c.Id, c);
        }
    }

    if (updatedLocationContacts.size() > 0) {
        List<Boat__c> relatedBoats = new List<Boat__c>([
            select Id, Contact__c, location__c from Boat__c where Contact__c IN :updatedLocationContacts.keySet()
        ]);

        for (Boat__c b : relatedBoats) {
            b.location__c = updatedLocationContacts.get(b.Contact__c).location_boat__c;
        }

        update boats;
    }
}