Always be lazy
Scott Bellware has an interesting post about Lazy Load in ORMs.
IMHO, I prefer eager load of all scalar values and load on demand (lazy load) for all complex objects, even when the cardinality is M..1 and 1..1. And once something is loaded, it stays loaded until until the user explicitly resets the navigation relationship. In other words, navigations cause load on first access and never again. This is how we have implemented Jasper - with two exceptions:
1. Jasper allows objects to be added to collections which have not been populated.
2. When a query is being built up on a collection (i.e. Northwind.Orders.Where().OrderDetails().OrderBy() ) query execution is delayed and the intermediary collections are not materialized.
Obviously, exception #2 is about the only way to support Linq and makes complete sense. Exception #1 is a bit more interesting in that the only reason for it is to allow inserting on objects for top level collections - in other words we didn't want to force the developer to load Northwind.Orders to add a single order. I personally don't think that is an important enough scenario to merit the exception, but the Jasper design team convinced me otherwise.
To be honest, I don't like Span (the ability to front load related objects) and think the N+1 query problem solves a demoware issue not a real life problem. In addition, Span introduces a bunch of complexity into the ORM framework which does not justify the benefit.
For Jasper, we wanted a single solution which was the most obvious and easiest to explain - which is consistent with our design principles. As a result, there will be apps where Jasper may not be the right solution - but that is that tax when using a prescriptive framework.
Not having Span can lead to situations for the in memory object graph is inconsistent with the database in strange and complex ways. However, I would argue that this is really a problem with the app logic that the framework is exposing. Span support just hides the problem and doesn't fix it unless the app developer is smart enough about using Span to manage their in-memory object graph - and that seems tricky at best for non-trivial apps.
That all said, we are open to adding Span support in Jasper depending on feedback.