1 files changed, 14 insertions, 0 deletions
diff --git a/entity/id.go b/entity/id.go
index 08916987..9e724012 100644
--- a/entity/id.go
+++ b/entity/id.go
@@ -1,6 +1,7 @@
 package entity
 
 import (
+	"crypto/sha256"
 	"fmt"
 	"io"
 	"strings"
@@ -17,6 +18,15 @@ const UnsetId = Id("unset")
 // Id is an identifier for an entity or part of an entity
 type Id string
 
+// DeriveId generate an Id from some data, taken from a root part of the entity.
+func DeriveId(data []byte) Id {
+	// My understanding is that sha256 is enough to prevent collision (git use that, so ...?)
+	// If you read this code, I'd be happy to be schooled.
+
+	sum := sha256.Sum256(data)
+	return Id(fmt.Sprintf("%x", sum))
+}
+
 // String return the identifier as a string
 func (i Id) String() string {
 	return string(i)
@@ -55,6 +65,10 @@ func (i Id) MarshalGQL(w io.Writer) {
 
 // IsValid tell if the Id is valid
 func (i Id) Validate() error {
+	// Special case to
+	if len(i) == 40 {
+		return fmt.Errorf("outdated repository format, please use https://github.com/MichaelMure/git-bug-migration to upgrade")
+	}
 	if len(i) != idLength {
 		return fmt.Errorf("invalid length")
 	}