Fix issue with DB ID not found in generator.

In the case there is a relation in the projection, but the related
assets are not in the projection, the assets are not pushed into the
registry leading to missing IDs when searching into it. In that case we
can simply return id -1 because the relation won't be displayed anyway
as we are missing one or both vertices.

Author: clems4ever

internal/database/id_generator.go

@@ -1,11 +1,15 @@
 package database
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/clems4ever/go-graphkb/internal/knowledge"
 )
 
+// ErrInexistentDBID represent an error thrown when a DB ID has not been pushed into th registry
+var ErrInexistentDBID = errors.New("DB ID does not exist")
+
 // AssetTemporaryIDGenerator is a kind of cache associating IDs to assets so that pivot points are joined by those IDs
 type AssetTemporaryIDGenerator struct {
 	// map the DB ID with a temporarily generated ID.
@@ -55,7 +59,7 @@ func (atig *AssetTemporaryIDGenerator) Push(asset knowledge.Asset, DBID int) (in
 func (atig *AssetTemporaryIDGenerator) Get(DBID int) (int, error) {
 	id, ok := atig.DBIDToTemporaryID[DBID]
 	if !ok {
-		return 0, fmt.Errorf("DB ID %d does not exist in generator", DBID)
+		return 0, ErrInexistentDBID
 	}
 	return id, nil
 }

internal/database/mariadb.go

@@ -658,12 +658,20 @@ func (mc *MariaDBCursor) Read(ctx context.Context, doc interface{}) error {
 
 			tmpIDFrom, err := mc.temporaryIDGenerator.Get(r.From)
 			if err != nil {
-				return fmt.Errorf("Unable to retrieve the temporary ID for DB ID %d (from): %v", r.From, err)
+				if err == ErrInexistentDBID {
+					tmpIDFrom = -1
+				} else {
+					return fmt.Errorf("Unable to retrieve the temporary ID for DB ID %d (from): %v", r.From, err)
+				}
 			}
 
 			tmpIDTo, err := mc.temporaryIDGenerator.Get(r.To)
 			if err != nil {
-				return fmt.Errorf("Unable to retrieve the temporary ID for DB ID %d (to): %v", r.To, err)
+				if err == ErrInexistentDBID {
+					tmpIDFrom = -1
+				} else {
+					return fmt.Errorf("Unable to retrieve the temporary ID for DB ID %d (to): %v", r.To, err)
+				}
 			}
 
 			output[i] = knowledge.RelationWithID{