How to get records for given contact id in this case?

Question

I created a custom object named Related_Staff_Positions which is a child lookup to the contact object.

Whenever one saves a new record, in a trigger I wish to get all records in the Related_Staff_Positions object for the given contact and the contact id itself.

Here is my skeleton code that does not work at all. What do I need to modify? Sorry but I am still new at this.

trigger trgr_Related_Staff_Position on Related_Staff_Positions__c (after insert,after update,after delete) 
{

//  Set<Id> setConID = new Set<Id>();
    List<Related_Staff_Positions__c> rsp_List = new List<Related_Staff_Positions__c>();
    rsp_LIst = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where Contact__c IN: Trigger.New];
    <ID> id = trigger.get(contact_c);
    if(Trigger.isInsert)
    {
        for(Related_Staff_Positions__c ct : Trigger.New)
        {
            ct.Contact__c;
        }
    }
    else if(Trigger.isUpdate) 
    {
        for(Related_Staff_Positions__c ct : Trigger.New)
        {

        }

    }
}

Answer 1

If you need to get the parent Contact id, you have to access the Contact__c of incoming child records. And your query filter should be set to filter from child Ids and not the Contact__c. So your initial query should be like below.

rsp_LIst = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where Id IN: Trigger.New];

And also your trigger should be bulkified as the best practice. Don't consider only one record, but all the incoming records in the trigger context. Iterate through the result list and collect the Contact Ids.

rsp_LIst = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where Id IN: Trigger.New];
List<String> contactIds = new List<String>();
for(Related_Staff_Positions__c rsp : Related_Staff_Positions__c){
    contactIds.add(rsp.Contact__c);
}

// then you can retrieve the other child records which mapped into the given contact
List<Related_Staff_Positions__c> otherChildren = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where Id IN: contactIds];

Answer 2

I am not sure why you are allowed to compile your code. Because this line:

rsp_LIst = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where Contact__c IN: Trigger.New];

would give the

Invalid Bind Variable error

Anyways, you can change the above line to search with the Related_Staff_Positions__c Id like this:

rsp_LIst = [Select Contact__c , first_name__c, last_name__c from Related_Staff_Positions__c where ID IN: Trigger.New];

You will get Contact ID from this list as:

rsp_LIst[0].Contact__c

Hope this helps!