Setting property to nil doesn't save to database

[Vortec4800]

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest versions of Parse Server and the Parse Swift SDK.

Issue Description

Setting a value on an optional model property doesn't propagate to the server.

Steps to reproduce

struct TestObject: ParseObject {
	// ParseObject Conformance
	var objectId: String?
	var createdAt: Date?
	var updatedAt: Date?
	var ACL: ParseACL?

	var testValue: String?
}

var testObj = TestObject()
testObj.testValue = "value"

testObj = try! testObj.save()
// Break here and check value on dashboard

testObj.testValue = nil

testObj = try! testObj.save()
// Break here and check value again on dashboard, see that it still says "value" and not (undefined)

Expected Outcome

Final value on dashboard should be (undefined) or nil, instead of keeping the old value.

Environment

Client

• Parse Swift SDK version: 2.0.2
• Xcode version: 13.0
• Operating system (iOS, macOS, watchOS, etc.): macOS
• Operating system version: 11.6

Server

• Parse Server version: 4.10.4
• Operating system: Ubuntu
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): Digital Ocean

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 4.4
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): Digital Ocean

[parse-github-assistant]

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[cbaker6]

The Swift SDK uses a tailored JSONEncoder. The Apple JSONEncoder ignores nil, so the behavior you mentioned is expected. You can see workarounds and comments about this:

• https://stackoverflow.com/questions/47266862/encode-nil-value-as-null-with-jsonencoder
• https://bugs.swift.org/browse/SR-9232
• A detailed explanation: https://newbedev.com/encode-nil-value-as-null-with-jsonencoder

The easiest way to achieve what you are looking for is by using ParseOperation, particularly unset. More in the SDK documentation. Every ParseObject has an operation available. This can be done two ways:

var testObj = TestObject()
let operations = testObj.operation.unset("testValue") // Unset on the server
testObj = try await operations.save()

// or unset on the server and on the local ParseObject
let operations = testObj.operation.unset(("testValue", \.testValue))
testObj = try await operations.save()

// or unset multiple during the same save on the server and locally
let operations = testObj.operation
  .unset(("testValue", \.testValue))
  .unset(("testValue2", \.testValue2))
testObj = try await operations.save()

If you need to access the custom ParseEncoder, see the post here: https://community.parseplatform.org/t/parsefile-init-without-previous-fetch/1666/6?u=cbaker6

You can also access the encoders using a static ParseObject, TestObject.getEncoder()

[Vortec4800]

I see. Given the movement in that Swift thread, it doesn't seem like this is going to be addressed by the Swift Encoder any time soon.

It does seem like this would be expected behavior for the SDK, given the behavior of other Parse SDKs and setting nil values. Unset works but we do have situations in some cases where values drop to nil without necessarily being explicitly unset.

The NullEncodable property wrapper referenced in that Stack Overflow question looks interesting, seems like it would handle the situation properly as long as optional model properties had this wrapper attached to it. How do you feel about including an implementation of this in the ParseSwift SDK and then including in the documentation instructions on using it for optional values? Realm SDK model objects use property wrappers quite a bit and it's a fairly easily understood process so I don't think that's out of the ordinary, and then it would clarify in the sample model objects that optional values need this wrapper - we weren't aware that the SDK didn't do this already and it caused some bugs when we transitioned from the Objective C SDK to this one on one of our projects.

[cbaker6]

How do you feel about including an implementation of this in the ParseSwift SDK and then including in the documentation instructions on using it for optional values?

maybe, if you are willing to work on a PR with testcases and it doesn’t break the current testcases, I can review it

[cbaker6]

It seems another approach a developer can take is simply add the dependency via SPM, https://github.com/g-mark/NullCodable

and then use the wrapper on your property.

struct TestObject: ParseObject {
	// ParseObject Conformance
	var objectId: String?
	var createdAt: Date?
	var updatedAt: Date?
	var ACL: ParseACL?

        @NullCodable var testValue: String?
  }

var testObj = TestObject()
testObj.testValue = "value" // Set
testObj = try await testObj.save()

testObj.testValue = nil // Unset
testObj = try await testObj.save()

or you can use ParseOperation, #264 (comment)

[Vortec4800]

So I ran into a problem, and it ties into discussion being had over in #242 and #263.

This is incompatible with emptyObject - when that runs all properties are nil so on save, the encoder tries to unset all properties.

If the work in #263 does end up getting merged, I can likely use that to compare and then only unset properties that were explicitly unset, but I do feel weary about the SDK having implicit persistence. I'll probably jump in on that PR as well. I think some of these features are very important, but a cache might end up being quite fragile.

[parse-github-assistant]

The label type:feature cannot be used in combination with type:improvement.