improve field deletion in collection

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### master
 [Full Changelog](https://github.com/parse-community/parse-server/compare/4.3.0...master)
+- IMPROVE: Optimized deletion of class field from schema by using an index if available to do an index scan instead of a collection scan. [#6815](https://github.com/parse-community/parse-server/issues/6815). Thanks to [Manuel Trezza](https://github.com/mtrezza).
 
 ### 4.3.0
 [Full Changelog](https://github.com/parse-community/parse-server/compare/4.2.0...4.3.0)

spec/MongoStorageAdapter.spec.js

@@ -351,6 +351,43 @@ describe_only_db('mongo')('MongoStorageAdapter', () => {
     expect(postIndexPlan.executionStats.executionStages.stage).toBe('FETCH');
   });
 
+  it('should delete field without index', async () => {
+    const database = Config.get(Parse.applicationId).database;
+    const obj = new Parse.Object('MyObject');
+    obj.set("test", 1);
+    await obj.save();
+    const schemaBeforeDeletion = await new Parse.Schema('MyObject').get();
+    await database.adapter.deleteFields(
+      "MyObject",
+      schemaBeforeDeletion,
+      ["test"]
+    );
+    const schemaAfterDeletion = await new Parse.Schema('MyObject').get();
+    expect(schemaBeforeDeletion.fields.test).toBeDefined();
+    expect(schemaAfterDeletion.fields.test).toBeUndefined();
+  });
+
+  it('should delete field with index', async () => {
+    const database = Config.get(Parse.applicationId).database;
+    const obj = new Parse.Object('MyObject');
+    obj.set("test", 1);
+    await obj.save();
+    const schemaBeforeDeletion = await new Parse.Schema('MyObject').get();
+    await database.adapter.ensureIndex(
+      'MyObject',
+      schemaBeforeDeletion,
+      ['test']
+    );
+    await database.adapter.deleteFields(
+      "MyObject",
+      schemaBeforeDeletion,
+      ["test"]
+    );
+    const schemaAfterDeletion = await new Parse.Schema('MyObject').get();
+    expect(schemaBeforeDeletion.fields.test).toBeDefined();
+    expect(schemaAfterDeletion.fields.test).toBeUndefined();
+  });
+
   if (
     semver.satisfies(process.env.MONGODB_VERSION, '>=4.0.4') &&
     process.env.MONGODB_TOPOLOGY === 'replicaset' &&

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -433,14 +433,19 @@ export class MongoStorageAdapter implements StorageAdapter {
       collectionUpdate['$unset'][name] = null;
     });
 
+    const collectionFilter = { $or: [] };
+    mongoFormatNames.forEach(name => {
+      collectionFilter['$or'].push({ [name]: { $exists: true } });
+    });
+
     const schemaUpdate = { $unset: {} };
     fieldNames.forEach((name) => {
       schemaUpdate['$unset'][name] = null;
       schemaUpdate['$unset'][`_metadata.fields_options.${name}`] = null;
     });
 
     return this._adaptiveCollection(className)
-      .then((collection) => collection.updateMany({}, collectionUpdate))
+      .then((collection) => collection.updateMany(collectionFilter, collectionUpdate))
       .then(() => this._schemaCollection())
       .then((schemaCollection) =>
         schemaCollection.updateSchema(className, schemaUpdate)