Fix nested Scopes, close go-gorm#4196

Author: jinzhu

callbacks.go

@@ -109,10 +109,12 @@ func (p *processor) Execute(db *DB) {
 	}
 
 	// call scopes
-	scopes := stmt.scopes
-	stmt.scopes = nil
-	for _, scope := range scopes {
-		db = scope(db)
+	for len(stmt.scopes) > 0 {
+		scopes := stmt.scopes
+		stmt.scopes = nil
+		for _, scope := range scopes {
+			db = scope(db)
+		}
 	}
 
 	for _, f := range p.fns {

migrator.go

@@ -8,10 +8,12 @@ import (
 // Migrator returns migrator
 func (db *DB) Migrator() Migrator {
 	// apply scopes to migrator
-	scopes := db.Statement.scopes
-	db.Statement.scopes = nil
-	for _, scope := range scopes {
-		db = scope(db)
+	for len(db.Statement.scopes) > 0 {
+		scopes := db.Statement.scopes
+		db.Statement.scopes = nil
+		for _, scope := range scopes {
+			db = scope(db)
+		}
 	}
 
 	return db.Dialector.Migrator(db.Session(&Session{}))