feat(rain): implement First, TxOptions, and relation column selection

pkg/rain/query_select.go

@@ -184,6 +184,7 @@ func (q *SelectQuery) WithRelations(names ...string) *SelectQuery {
 type RelationConfig struct {
 	Where   schema.Predicate
 	OrderBy []schema.OrderExpr
+	Columns []schema.Expression
 }
 
 // Relation configures filters and ordering for a named relation.
@@ -629,6 +630,12 @@ func (q *SelectQuery) writeJoins(ctx *compileContext) error {
 	return nil
 }
 
+// First executes the SELECT query with an implicit LIMIT 1 and scans the first row into dest.
+// Returns sql.ErrNoRows if no result is found.
+func (q *SelectQuery) First(ctx context.Context, dest any) error {
+	return q.clone().Limit(1).Scan(ctx, dest)
+}
+
 // Scan executes the SELECT query and scans results into dest.
 func (q *SelectQuery) Scan(ctx context.Context, dest any) error {
 	if q.runner == nil {

pkg/rain/rain.go

@@ -279,12 +279,17 @@ func (db *DB) QueryRow(ctx context.Context, query string, args ...any) *sql.Row
 
 // Begin starts a new transaction.
 func (db *DB) Begin(ctx context.Context) (*Tx, error) {
+	return db.BeginTx(ctx, nil)
+}
+
+// BeginTx starts a new transaction with the provided options.
+func (db *DB) BeginTx(ctx context.Context, opts *sql.TxOptions) (*Tx, error) {
 	primary := db.primaryHandle()
 	if primary.db == nil {
 		return nil, ErrNoConnection
 	}
 
-	tx, err := primary.db.BeginTx(ctx, nil)
+	tx, err := primary.db.BeginTx(ctx, opts)
 	if err != nil {
 		return nil, err
 	}
@@ -294,7 +299,12 @@ func (db *DB) Begin(ctx context.Context) (*Tx, error) {
 
 // RunInTx executes fn in a transaction, rolling back on error and committing on success.
 func (db *DB) RunInTx(ctx context.Context, fn func(*Tx) error) error {
-	tx, err := db.Begin(ctx)
+	return db.RunInTxOpts(ctx, nil, fn)
+}
+
+// RunInTxOpts executes fn in a transaction with the provided options, rolling back on error and committing on success.
+func (db *DB) RunInTxOpts(ctx context.Context, opts *sql.TxOptions, fn func(*Tx) error) error {
+	tx, err := db.BeginTx(ctx, opts)
 	if err != nil {
 		return err
 	}

pkg/rain/relation_loading.go

@@ -295,6 +295,7 @@ func (q *SelectQuery) loadRelatedManyToManyRows(
 
 		batchDest := reflect.New(reflect.SliceOf(relatedElemType))
 		targetQuery := &SelectQuery{runner: q.runner, dialect: q.dialect, table: tableDefSource{table: relation.TargetTable}}
+		ensureTargetColumnSelected(targetQuery, config.Columns, relation.TargetColumn)
 		if config.Where != nil {
 			targetQuery.Where(config.Where)
 		}
@@ -376,6 +377,7 @@ func (q *SelectQuery) loadRelatedRows(
 		end := min(start+relationBatchSize, len(sourceKeys))
 		batchDest := reflect.New(reflect.SliceOf(relatedElemType))
 		query := &SelectQuery{runner: q.runner, dialect: q.dialect, table: tableDefSource{table: relation.TargetTable}}
+		ensureTargetColumnSelected(query, config.Columns, relation.TargetColumn)
 		if config.Where != nil {
 			query.Where(config.Where)
 		}
@@ -544,6 +546,26 @@ func setRelationValue(parent reflect.Value, relationName string, relationType sc
 	}
 }
 
+func ensureTargetColumnSelected(query *SelectQuery, columns []schema.Expression, targetCol *schema.ColumnDef) {
+	if len(columns) == 0 {
+		return
+	}
+
+	query.Column(columns...)
+
+	// Ensure mapping target column is selected.
+	found := false
+	for _, col := range columns {
+		if cr, ok := col.(schema.ColumnReference); ok && cr.ColumnDef() == targetCol {
+			found = true
+			break
+		}
+	}
+	if !found {
+		query.Column(schema.Ref(targetCol))
+	}
+}
+
 func dereferenceModelValue(value reflect.Value) reflect.Value {
 	current := value
 	for current.IsValid() && current.Kind() == reflect.Pointer {