fix(mysql): improve unique key violation handling (#9724)

lib/dialects/mysql/query.js

@@ -204,13 +204,15 @@ class Query extends AbstractQuery {
         let fields = {};
         let message = 'Validation error';
         const values = match ? match[1].split('-') : undefined;
-        const uniqueKey = this.model && this.model.uniqueKeys[match[2]];
+        const fieldKey = match ? match[2] : undefined;
+        const fieldVal = match ? match[1] : undefined;
+        const uniqueKey = this.model && this.model.uniqueKeys[fieldKey];
 
         if (uniqueKey) {
           if (uniqueKey.msg) message = uniqueKey.msg;
           fields = _.zipObject(uniqueKey.fields, values);
         } else {
-          fields[match[2]] = match[1];
+          fields[fieldKey] = fieldVal;
         }
 
         const errors = [];