Hmmm, I don't want to assume too much (because of course everyone's project needs are different), but that just sounds like a terrible idea. I guess there are solutions like goldiloader (I've never tried it), but 99% of the time I'd rather not load associated records unless I use them. When I need to load/use them along with many parent records, it seems pretty obvious that I'll want to include those associations (eagerly loaded) in my AR query to avoid N+1 queries as you mentioned. Then again, maybe I've just spent too long taking those assumptions for granted where newer devs might not.
EDIT: After looking more into your claim that "N+1 queries aren't possible with Ecto", I think I have a better idea for what you might mean. Perhaps you don't want everything eager loaded, but you want an exception to be raised if you try to access an associated record that hasn't been preloaded. I suppose that's a fair point (probably good practice if having any N+1 queries will be a major problem in your project, or if subpar performance really is your biggest threat), and no, I don't know of a way to do that in AR.