How to get child objects from Junction object

Question

I have a list of VisualForce components called VFComponent, a list of attributes name VFAttributes and a junction object VFComponentToAttribute that links them in a many to many relationship.

I am trying to get a list of components, in the form of:

List<VFComponent__c> VList = [Query here FROM VFComponentToAttribute];

For the query I have tried getting Id, VFComponent__r.Id, VFComponent all to no avail. Everything I try seems to give me a type error.

Could someone be kind enough to explain how I could get a list,populated with child objects from the junction object in this way.

To clarify I am trying to pass a list of objects to some other code, but in general I would like to better understand what Salesforce actually stores in the junction object, I thought is was something like a key, but if I query the Id and try to use that I get type mismatches as well:

List<Id> idList = [Select childobject__r.id From JuncitonObject];

That gives me also gives an error as if the query result were objects and not IDs, which I suppose they are?

In short I am confused.

Answer 1

Okay, this was due to my own ignorance (well I knew that going in or I would not have posted here).

Anyhow, the problem is that any query on an object returns a list of that object. You then have to get the list of identifiers into a separate list using a loop.

So for instance the following works to run another class on the VFComponent object using a list of Ids for VFComponent from the VFComponentToAttribute object:

String compname = '%apex:pageBlock%';
List<Id> idList = new List<Id>();
List<VFComponentToAttributes__c> complist = [SELECT Id, Name, VFComponent__r.Id FROM VFComponentToAttributes__c Where VFComponent__r.Name Like :compname];                 
for(VFComponentToAttributes__c c: complist){  
    System.debug('c.name= ' + c.name + ', c.Id= ' + c.Id);
    idList.add(c.VFComponent__r.Id);
}
UpdateComponentJSON.UpdateJSON(idList);

But if anyone else would like to suggest a more elegant solution I would be glad to hear it.

Also is there any way to find the true structure of the underlying database that Salesforce uses? I think this would be a lot less confusing if I knew what the code actually mapped to.

Answer 2

There are few standard junction objects in SF. Let's check Quote

It has accountid and OpportunityId, we can get opp name if we know account id

Get Opportunity Ids:

select OpportunityId from quote where accountid = '00155000005BXMYAA4'

Then get Opportunity Name:

select name from Opportunity where id in (select OpportunityId from quote where accountid = '00155000005BXMYAA4')

You could go to schema and check your objects, or you can go to 'Setup - Create - Object' and see for relashionships

Select account id for opportunity name:

select accountid from Quote where opportunityid in (select id from opportunity where name like '%BigData%')