Add R.Loaded to track which relationships have been loaded

[jacobmolby]

Why

Today, the only way to inspect whether a generated relationship has been loaded is to look at the value in R. That collapses two distinct states into one:

• A to-one relation: model.R.Pilot == nil could mean "never loaded" or "loaded, no related row".
• A to-many relation: len(model.R.Jets) == 0 could mean "never loaded" or "loaded, zero rows" (loaders reset to nil and then append, so the zero-row case is indistinguishable from never-loaded).

Callers have no reliable way to tell these apart, which is a real footgun when the FK is nullable or the related set may
legitimately be empty.

Solution

A new typed struct, nested inside R as R.Loaded, with one bool per relationship.

if pilot.R.Loaded.Airport && pilot.R.Airport == nil {
    // definitively no airport
}

if pilot.R.Loaded.Jets && len(pilot.R.Jets) == 0 {
    // definitively zero jets
}

R.Loaded.X is set to true by:

• LoadX, Preload.X, and ThenLoad.X after they populate R.X (including the zero-row case).
• The inverse-side assignment performed during loading (e.g. pilots.LoadJets(...) flips jet.R.Loaded.Pilot on each child).
• AttachX / InsertX for to-one relations, where the relation is fully known after the call.
• Factory Build and Create

R.Loaded.X is not changed by AttachX / InsertX for to-many relations, since appending rows does not turn a partial slice into a complete one.

Behaviour change: slice LoadX now resets stale to-one values

In 110_loaders.go.tpl, the per-parent reset loop in both slice-load paths previously ran only for to-many relations - slice LoadX on a to-one relation would leave a stale R.X in place if the new query returned no match for that parent. Single-object LoadX already reset, so the two paths were inconsistent.

We remove the {{if $rel.IsToMany}} guard around that reset, so slice LoadX on a to-one now also clears R.X per parent before the inner match loop. This is required for the new R.Loaded contract to hold - without it, a parent could end up withLoaded.X == true while R.X still points at a row from an earlier load.

Other Notes

• The relationship alias Loaded is now reserved; generation fails with a clear error if any relationship is aliased that way.
• Hand-mutating R directly (e.g. jet.R.Pilot = pilot) does not update R.Loaded

[stephenafamo]

I'm not sure about this, because it reserves a relationship name

[jacobmolby]

I can make the "Loaded" name configurable in the bobgen config, so users can change it themselves, rather than aliasing their own relations in case of conflict?

[stephenafamo]

Alright, I like that idea 👍🏾

[jacobmolby]

It's updated now :)