Why doesn't Deletion in Parse-server require to obtain that object beforehand?

Question

I came across a strange behaviour. Even though my unit tests are passing, I don;t quite understand this.

func testDeleteMoment() {
        // Create a Moment for current user
        let expectCreate = expectation(description: "create moment should succeed")
        Datastore.shared.createMoment(notes: "myNotes", rating: 1, time: Date(), location: "", isPublic: true, completion: {(success, error) in
            XCTAssertTrue(success)
            expectCreate.fulfill()
        })
        waitForExpectations(timeout: 5) { (error) in
            XCTAssertNil(error, "Test timed out. \(String(describing: error?.localizedDescription))")
        }
        let query = PFQuery(className: "Moment")
        var objectId = ""
        do {
            query.whereKey("owner", equalTo:PFUser.current()!)
            let object = try query.getFirstObject()
            objectId = object.objectId!
        } catch {}
        let task = Datastore.shared.deleteMoment(id: objectId)
        task.waitUntilFinished()
        let query2 = PFQuery(className: "Moment")
        query2.whereKey("owner", equalTo:PFUser.current()!)
        let task3 = query2.countObjectsInBackground()
        task3.waitUntilFinished()
        XCTAssertEqual(task3.result, 0)
    }

While writing my datastore.deleteMoment(), I noticed that unlike saveEventually(), deleteEventually() doesn't have a completion handler. Instead, it only comes with a BFTask<NSNumber>. Hence I experimented with the following code and the unit test passes to my surprise.

func deleteMoment(id: String) -> BFTask<NSNumber> {
        let pfMoment = PFObject(className:"Moment")
        pfMoment.objectId = id
        return pfMoment.deleteEventually()
    }

How comes that I don't have to retrieve the object before hand in order to delete it? Why isn't there then just a method to delete the object via an id, instead of doing it like this?

Answer 1

How comes that I don't have to retrieve the object before hand in order to delete it?

Parse lets you work with "shell objects". Basically, if you know the object id you can attach it to a blank instance of a Parse Object subclass and do some actions on it. You can create a shell, assign a couple values, and call object.save() and it will update just those fields. You can create a blank object, assign an id, then call object.fetch() to obtain it, without having to have gotten the object from a query or pointer on another object.

The reason it works is because in order to perform the database operations, the object id and class name are the only pieces of information required. If you just want to update a couple fields, you don't need to pull all of the rest of the data to do so. And if you want to destroy an object, why would you need all of it's data locally first? You just need to remove the entry in the database with the matching _id.

Why isn't there then just a method to delete the object via an id, instead of doing it like this?

Nobody has built it yet. Parse is open source, and while it's a pretty phenomenal resource today, there's certainly a lot of room for improvement. Feel free to add this feature and create a PR. Though you could easily build your own wrapper by extending the Parse.Object class with this function, that basically does what you already did under the hood.