dialect/sql: add Joined<T> helpers for Selector to avoid double joining (#3419)

dialect/sql/builder.go

@@ -2388,11 +2388,11 @@ func (s *Selector) Table() *SelectTable {
 }
 
 // selectTable returns a *SelectTable from the given TableView.
-func selectTable(tb TableView) *SelectTable {
-	if tb == nil {
+func selectTable(t TableView) *SelectTable {
+	if t == nil {
 		return nil
 	}
-	switch view := tb.(type) {
+	switch view := t.(type) {
 	case *SelectTable:
 		return view
 	case *Selector:
@@ -2400,8 +2400,10 @@ func selectTable(tb TableView) *SelectTable {
 			return nil
 		}
 		return selectTable(view.from[0])
+	case *queryView, *WithBuilder:
+		return nil
 	default:
-		panic(fmt.Sprintf("unhandled TableView type %T", tb))
+		panic(fmt.Sprintf("unexpected TableView %T", t))
 	}
 }
 
@@ -2422,6 +2424,38 @@ func (s *Selector) HasJoins() bool {
 	return len(s.joins) > 0
 }
 
+// JoinedTable returns the first joined table with the given name.
+func (s *Selector) JoinedTable(name string) (*SelectTable, bool) {
+	for _, j := range s.joins {
+		if t := selectTable(j.table); t != nil && t.name == name {
+			return t, true
+		}
+	}
+	return nil, false
+}
+
+// JoinedTableView returns the first joined TableView with the given name or alias.
+func (s *Selector) JoinedTableView(name string) (TableView, bool) {
+	for _, j := range s.joins {
+		switch t := j.table.(type) {
+		case *SelectTable:
+			if t.name == name || t.as == name {
+				return t, true
+			}
+		case *Selector:
+			if t.as == name {
+				return t, true
+			}
+			for _, t2 := range t.from {
+				if t3 := selectTable(t2); t3 != nil && (t3.name == name || t3.as == name) {
+					return t3, true
+				}
+			}
+		}
+	}
+	return nil, false
+}
+
 // Join appends a `JOIN` clause to the statement.
 func (s *Selector) Join(t TableView) *Selector {
 	return s.join("JOIN", t)