Avoid making a database query when updating a single document by _id

Author: alisnic

lib/mongo/Mutator.js

@@ -139,6 +139,9 @@ export default class Mutator {
         ...findOptions,
         fields: {},
       }).fetchAsync();
+    } else if (Object.keys(selector).length === 1 && typeof selector._id === "string") {
+      // if we are updating a single document by id, we can skip the find
+      docs = [{ _id: selector._id }];
     } else {
       docs = await this.find(selector, findOptions).fetchAsync();
     }