[SalesForce] URLFOR Action to New Object for specific recordtype

You can use the key prefix in conjunction with the RecordType parameter in a custom controller or extension. The basic idea is below. The developer name should be stable and not change between orgs. Also, once in production the ID doesn't change in sandboxes refreshed from production, so if you are already working with a record type that is in production you could use the ID.

You can adapt this code to be more specific or to not be an extension. The main thing is the format of the URL and the idea that the RecordType ID is retrieved in a relatively stable way.

public class BypassRecTypeController {
    private sObject rec;
    private String recTypeDevName;

    public BypassRecTypeController(ApexPages.StandardController controller) {
        rec = controller.getRecord();

        // might get this some other way...
        recTypeDevName = ApexPages.currentPage().getParameters().get('recTypeDevName');
    }

    public PageReference bypass() {
        // Get the key prefix to use as part of the url
        // Alternatively, if you know the SObject you could have something like
        // Account.SObjectType.getDescribe() hard coded.
        Schema.DescribeSObjectResult dsr = rec.getSObjectType().getDescribe();
        String keyPrefix = dsr.getKeyPrefix();

        // Get the record type to add on to the query string
        Id recTypeId = [Select Id From RecordType Where DeveloperName = :recTypeDevName].Id;

        PageReference pageRef = new PageReference('/' + keyPrefix + '/e?RecordType=' + recTypeId);

        return pageRef;
    }

}

Even metadata API won't help you in finding field Ids ;)

For custom fields (like a lookup to your parent object) they're just Ids you see in the URL when you navigate in setup to the field definition. For standard fields - it's easiest to just use Firebug or similar tool that lets you inspect HTML and learn name attributes of fields on standard page layouts. (this works on custom fields too, just sounds like bit more work when the Id is sitting right there in the URL...)

There's excellent blog post by Ray Dehler to get you started. Another useful link lists all standard fields.

Now, whether you should do it... Field Id will be different when you deploy the field from sandbox to prod. It will get stable once the field got deployed to production and you refresh all sandboxes from prod (like record types or reports if you've ever noticed that). If the field is already there - you could hardcode it and the URLFOR-like solution might be really a way to go ;) It's a bit fragile though and forget about it if you'll ever plan to package your custom buttons... You could store it in some custom setting I guess?

A more sane approach would be to create a VF page & controller extension and then make several buttons that link to it. Something like /apex/newChildObjectPage?parent=001...&recordTypeName=rec1 will be more readable later in maintenance. Painful to have to recreate the new/edit page from scratch though...