Generate set of Ids from a map, am I doing it right?

Question

I'm trying to generate a set of Opportunity Ids that have 'Legal' cases. But I'm getting the 'inner select field 'Opportunity__r.Id' cannot have more than one level of relationships. Is there a better way of doing this without first pulling a list of sObjects then iterating using a for loop to get the Ids? Thank for helping out!

Set<Id> oppIds = new Map<Id, Opportunity>([SELECT Id FROM Opportunity WHERE     Id IN (SELECT Opportunity__r.Id FROM Case WHERE RecordType.Name = 'Legal')]);

Answer 1

Change __r.Id to __c. That way it does not resolve to an object but just an Id.

Set<Id> oppIds = new Map<Id, Opportunity>([
    SELECT Id FROM Opportunity
    WHERE Id IN (
        SELECT Opportunity__c
        FROM Case
        WHERE RecordType.Name = 'Legal'
    )
]).keySet().clone();