Added support for subqueries in Where and Having clauses

Queries can be transformed into subqueries by calling .Subquery() on a db object

See main_test.go:TestQueryBuilderSubselectInWhere

main.go

@@ -168,6 +168,14 @@ func (s *DB) NewScope(value interface{}) *Scope {
 	return &Scope{db: dbClone, Search: dbClone.search.clone(), Value: value}
 }
 
+// Subquery returns the query as eprx object
+func (s *DB) Subquery() *expr {
+	scope := s.NewScope(s.Value)
+	scope.prepareQuerySQL()
+
+	return Expr("("+scope.SQL+")", scope.SQLVars...)
+}
+
 // Where return a new relation, filter records with given conditions, accepts `map`, `struct` or `string` as conditions, refer http://jinzhu.github.io/gorm/crud.html#query
 func (s *DB) Where(query interface{}, args ...interface{}) *DB {
 	return s.clone().search.Where(query, args...).db

main_test.go

@@ -607,6 +607,32 @@ func TestHaving(t *testing.T) {
 	}
 }
 
+func TestQueryBuilderSubselectInWhere(t *testing.T) {
+	user := User{Name: "user1", Email: "root@user1.com", Age: 32}
+	DB.Save(&user)
+	user = User{Name: "user2", Email: "nobody@user2.com", Age: 16}
+	DB.Save(&user)
+	user = User{Name: "user3", Email: "root@user3.com", Age: 64}
+	DB.Save(&user)
+	user = User{Name: "user4", Email: "somebody@user3.com", Age: 128}
+	DB.Save(&user)
+
+	var users []User
+	DB.Select("*").Where("name IN ?", DB.
+		Select("name").Table("users").Where("email LIKE ?", "root@%").Subquery()).Find(&users)
+
+	if len(users) != 2 {
+		t.Errorf("Two users should be found, instead found %d", len(users))
+	}
+
+	DB.Select("*").Where("age >= ?", DB.
+		Select("AVG(age)").Table("users").Subquery()).Find(&users)
+
+	if len(users) != 2 {
+		t.Errorf("Two users should be found, instead found %d", len(users))
+	}
+}
+
 func DialectHasTzSupport() bool {
 	// NB: mssql and FoundationDB do not support time zones.
 	if dialect := os.Getenv("GORM_DIALECT"); dialect == "mssql" || dialect == "foundation" {

scope.go

@@ -256,8 +256,12 @@ func (scope *Scope) AddToVars(value interface{}) string {
 	if expr, ok := value.(*expr); ok {
 		exp := expr.expr
 		for _, arg := range expr.args {
+			if pgParameterRegexp.MatchString(exp) {
+				exp = pgParameterRegexp.ReplaceAllLiteralString(exp, scope.AddToVars(arg))
+			} else {
 				exp = strings.Replace(exp, "?", scope.AddToVars(arg), 1)
 			}
+		}
 		return exp
 	}
 
@@ -452,6 +456,7 @@ var (
 	isNumberRegexp      = regexp.MustCompile("^\\s*\\d+\\s*$")                   // match if string is number
 	comparisonRegexp    = regexp.MustCompile("(?i) (=|<>|>|<|LIKE|IS|IN) ")
 	countingQueryRegexp = regexp.MustCompile("(?i)^count(.+)$")
+	pgParameterRegexp   = regexp.MustCompile(`\$[0-9]+`)                         // to exchange postgres `$1` style parameter placeholders
 )
 
 func (scope *Scope) quoteIfPossible(str string) string {