add method on models/Tag.js

src/server/models/tag.js

@@ -19,8 +19,16 @@ schema.plugin(mongoosePaginate);
  */
 class Tag {
 
-  async findOrCreate(tagName) {
-
+  static async findOrCreate(tagName) {
+    const tag = await this.findOne({
+      name: tagName
+    });
+    if (!tag) {
+      return await this.create({
+        name: tagName
+      });
+    }
+    return tag;
   }
 
 }