Reverse edge for history entity is generated in the wrong direction

[mdreizin]

Follow-up to #81.

Describe the bug

When WithReverseEdge is enabled, the generated EventHistory edge is inverted. It uses edge.To("event", Event.Type) which makes ent generate extra FK fields/constraints on the history table. The reverse edge should be defined using edge.From(...).Ref(...).Field("ref") to attach to the existing ref field.

To Reproduce
Minimal schema:

// EventHistory schema
func (EventHistory) Edges() []ent.Edge {
	return []ent.Edge{
		// Generated by extension (currently)
		edge.To("event", Event.Type).Unique(),
	}
}

// Event schema
func (Event) Edges() []ent.Edge {
	return []ent.Edge{
		edge.To("history", EventHistory.Type),
	}
}

Generator setup:

enthistory.Generate(
	// ...
	enthistory.WithReverseEdge(),
)

entc.Generate(
	// ...
	enthistory.NewHistoryExtension(
		enthistory.WithReverseEdgeExtension(),
	),
)

Expected behavior
The generated EventHistory edge should be:

edge.From("event", Event.Type).
	Ref("history").
	Field("ref").
	Unique().
	Immutable()

This keeps the FK on event_history.ref and avoids ent generating unnecessary fields/constraints.

Screenshots
N/A

Additional context
This only happens when WithReverseEdge() and WithReverseEdgeExtension() are used together.

[andrecastillo]

Thanks for reporting this. I investigated and here is what I found:

The symptom is accurate

You are right that reverseEdge() in generate.go creates an edge.Descriptor without setting Inverse: true, so the schemast serializer emits edge.To() instead of edge.From(). This causes ent to generate a separate FK column on each history table (e.g., character_history_character) alongside the existing ref field -- both storing the same entity ID.

However, the suggested fix would break history queries for deleted entities

Using edge.From(...).Ref("history").Field("ref") would make ref a FK column with ON DELETE SET NULL. When the original entity is deleted, the database would set ref to NULL. All history query methods (History(), Latest(), Earliest(), AsOf(), Diff()) filter by ref, so they would stop finding DELETE-operation history records -- which defeats the purpose of an audit system.

The two-column approach is intentional

The current design keeps ref as a plain field (always preserves the original entity ID, even after deletion) and uses the edge FK column separately for ent traversal (which correctly becomes NULL when the entity is deleted). This is the trade-off mentioned in the code comment at generate.go:563-565.

What IS wrong

1. Edge direction -- edge.To() on the history schema makes it the edge "owner," which is semantically backwards. It should be edge.From().
2. Example is incomplete -- The WithReverseEdge() doc says the original schema must define a matching edge.To("history", ...), but the basic example does not include it.

The correct fix would be changing to edge.From() WITHOUT .Field("ref") (keeping the separate FK column intentionally), and updating examples to include the required edge on the original schema. But this would be a breaking change for existing WithReverseEdge users since it changes the generated schema structure.