feat: disciple link

internal/repository/disciple_repo.go

@@ -2,6 +2,7 @@ package repository
 
 import (
 	"context"
+	"errors"
 	"time"
 
 	"go.mongodb.org/mongo-driver/v2/bson"
@@ -88,6 +89,60 @@ func (r *DiscipleRepo) GetByLetterAndSuffix(ctx context.Context, letter *string,
 	return results, nil
 }
 
+// FindByIDs returns summary docs for the given ObjectID hex strings (order
+// unspecified). Ids that aren't valid ObjectIDs are skipped rather than
+// failing the batch — the ids come from a Postgres column with no FK, so a
+// malformed or stale value must degrade gracefully. An empty/all-invalid
+// input short-circuits without hitting Mongo.
+//
+// Used to hydrate disciple names onto admin user rows: the user→disciple link
+// lives in Postgres while the disciple record lives here.
+func (r *DiscipleRepo) FindByIDs(ctx context.Context, ids []string) ([]DiscipleSummaryDoc, error) {
+	oids := make([]bson.ObjectID, 0, len(ids))
+	for _, id := range ids {
+		if oid, err := bson.ObjectIDFromHex(id); err == nil {
+			oids = append(oids, oid)
+		}
+	}
+	if len(oids) == 0 {
+		return []DiscipleSummaryDoc{}, nil
+	}
+	cursor, err := r.col.Find(ctx,
+		bson.M{"_id": bson.M{"$in": oids}},
+		options.Find().SetProjection(discipleSummaryProjection),
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer cursor.Close(ctx)
+	var results []DiscipleSummaryDoc
+	if err := cursor.All(ctx, &results); err != nil {
+		return nil, err
+	}
+	return results, nil
+}
+
+// FindByID returns one disciple summary, or nil when the id is malformed or
+// no such document exists. Callers use it to validate a link target before
+// writing it to Postgres.
+func (r *DiscipleRepo) FindByID(ctx context.Context, id string) (*DiscipleSummaryDoc, error) {
+	oid, err := bson.ObjectIDFromHex(id)
+	if err != nil {
+		return nil, nil // malformed id — "not found", not an error
+	}
+	var doc DiscipleSummaryDoc
+	err = r.col.FindOne(ctx, bson.M{"_id": oid},
+		options.FindOne().SetProjection(discipleSummaryProjection),
+	).Decode(&doc)
+	if errors.Is(err, mongo.ErrNoDocuments) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &doc, nil
+}
+
 func (r *DiscipleRepo) Count(ctx context.Context) (int64, error) {
 	return r.col.CountDocuments(ctx, bson.M{})
 }