dialect/sql: return selector's alias as a table name (#1745)

In case selector's from clause has *Selector instead of *SelectTable,
TableName() function (used for column checking) will panic.

Instead, return selector's alias which is useful if the query returns
a table with the same schema as some entity. Essentially, this allows
materialized views to reuse the same entity code as long as columns
are matching.

dialect/sql/builder.go

@@ -1946,9 +1946,16 @@ func (s *Selector) Table() *SelectTable {
 	return s.from.(*SelectTable)
 }
 
-// TableName returns the name of the selected table.
+// TableName returns the name of the selected table or alias of selector.
 func (s *Selector) TableName() string {
-	return s.Table().name
+	switch view := s.from.(type) {
+	case *SelectTable:
+		return view.name
+	case *Selector:
+		return view.as
+	default:
+		panic(fmt.Sprintf("unhandled TableView type %T", s.from))
+	}
 }
 
 // Join appends a `JOIN` clause to the statement.