Cross-Referencing a Custom Object's Field Directly in a Condition Using Apex Class and Apex Trigger

Question

Say I have a custom object named Book__c with fields Title__c and a checkbox named New__c, and another custom object named Author__c with field Book__c, can I use New__c field directly with Author__c object in a condition?

The requirement is to automatically create a new Obj_To_Insert__c record when a new Author__c record with Book__c field having its field New__c set to true.

For example, can I use it like this?

List<Author__c> authorList = new List<Author__c>

for(Author__c auth : authorList) {
    if(auth.Book__r.New__c == true) {
        //some code...
    }
} 

The code above is a snippet from this apex class:

public class ThisIsTheClass {

    public static void theMethod(List<Author__c> authorList) {

        Set<Id> id = new Set<Id>();
        List<Obj_To_Insert__c> otiList = new List<Obj_To_Insert__c>();

        authorList = [SELECT Id, Name, Book__c, Book__r.New__c FROM Author__C where Id IN: id];

        for(Author__c auth : authorList) {
            if(auth.Book__r.New__c == true) {
                Obj_To_Insert__c oti = new Obj_To_Insert__c();
                oti.Field1__c = auth.Id;
                otiList.add(oti);    
            } 
        } 

        insert otiList;

    }
}

Meanwhile, the apex class is called in the apex trigger below:

trigger thisIsTheTrigger on Author__c (after insert) {

    ThisIsTheClass.theMethod(Trigger.New);     

}

Answer 1

Your Author__c query cannot possibly return any records. You instantiate a Set<Id> and never populate it with anything. Basically, your filter is equivalent to:

WHERE Id IN ()

This filter will never return any results. Instead of using this Set, simply filter on the List of records you already have:

WHERE Id IN :authorList

If you really want to get the Set<Id> corresponding to authorList, the correct methodology is:

Set<Id> recordIds = new Map<Id, Author__c>(authorList).keySet();