dialect/sql/sqlgraph: add constraint error checks (#1316)

Initial work on #1310

dialect/sql/sqlgraph/errors.go

@@ -0,0 +1,48 @@
+// Copyright 2019-present Facebook Inc. All rights reserved.
+// This source code is licensed under the Apache 2.0 license found
+// in the LICENSE file in the root directory of this source tree.
+
+package sqlgraph
+
+import (
+	"errors"
+	"strings"
+)
+
+// IsConstraintError returns true if the error resulted from a DB constraint violation
+func IsConstraintError(err error) bool {
+	var e *ConstraintError
+	return errors.As(err, &e) || IsUniqueConstraintError(err) || IsForeignKeyConstraintError(err)
+}
+
+// IsUniqueConstraintError reports if the error resulted from a DB uniqueness constraint violation.
+// e.g. duplicate value in unique index.
+func IsUniqueConstraintError(err error) bool {
+	uniquenessErrors := []string{
+		"Error 1062",                 // MySQL
+		"violates unique constraint", // Postgres
+		"UNIQUE constraint failed",   // SQLite
+	}
+	for _, s := range uniquenessErrors {
+		if strings.Contains(err.Error(), s) {
+			return true
+		}
+	}
+	return false
+}
+
+// IsForeignKeyConstraintError reports if the error resulted from a DB FK constraint violation.
+// e.g. parent row does not exist.
+func IsForeignKeyConstraintError(err error) bool {
+	fkErrors := []string{
+		"Error 1452",                      // MySQL
+		"violates foreign key constraint", // Postgres
+		"FOREIGN KEY constraint failed",   // SQLite
+	}
+	for _, s := range fkErrors {
+		if strings.Contains(err.Error(), s) {
+			return true
+		}
+	}
+	return false
+}