How to check if a child object is null in Javascript Button?

Question

I'm creating a javascript button on the Quote object that would redirect to a link in another window. I only want to redirect to this other window, if there's a child under the record. For this example: a quote (master) can have multiple services (detail). The object name for services is QService__c.

I want to be able to do an if statement and see if Quote__c (name of the master-detail relationship field on the QService__c object) is not null. As well, I want to pass into the URL the id for the Quote record. Calling the SObject.id - is that the best way to pass that through? Here is what I have so far.

{!REQUIRESCRIPT("/SOAP/ajax/36.0/connection.js")}
var qt = new sforce.SObject("QService__c");
qt.id = "{!QService__c.id}";

if({!QService__c.Quote__c} != null) {
//Open up survey link in new window
window.open("https://services.qualtrics.com/jfe/form/SV_9o7r0gk556N0bGZ/?={!QService__c.id});} 
else {
//Catch error
alert("There needs to be a service on this quote."); }

Answer 1

You cannot simply reference child objects with a Javascript button. You will need to do a SOQL query to see if they exist. You can do a SOQL query from within a JS button.

{!RequireScript("/soap/ajax/42.0/connection.js")}
var childRecords = sforce.connection.query("SELECT Id from Quote__c where QService__c ='{!QService__c.Id}'");

Then, you can check to see if you got any record back:

if(childRecords.getArray("records").length) {
   window.open("https://services.qualtrics.com/jfe/form/SV_9o7r0gk556N0bGZ/?={!QService__c.id}");
} else {
    alert("There needs to be a service on this quote."); 
}

Edit: Here is a link to an example of what I'm suggesting on the developer.salesforce forums.

https://developer.salesforce.com/forums/?id=906F0000000BPbaIAG

Answer 2

So I ended up doing a completely different solution than what was suggested here (even though I believe these could've done the trick). What I ended up doing was:

1. Fixing my query. I was doing that all wrong. I needed to do a Child-to-parent query.
2. Use .size and check against if the size of the array was 0.

Here's the code:

{!RequireScript("/soap/ajax/42.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/28.0/apex.js")} 

var quoteId = "{!Quote.Id}"; 

alert("This is the quote Id - " + quoteId); 

var childRecords = sforce.connection.query("SELECT id, Name, Quote__r.Id FROM QService__c WHERE Quote__r.id ='"+ quoteId +"'"); 

alert("The # of child records is/are: " + childRecords.size); 

if(childRecords.size==0) { 
alert("There needs to be a service on this quote."); 
} else { 
window.open("http://www.krusecollins.com/") 
}

I hope this helps others, and thanks for everyone who helped on this.