Save pointer to new object in beforeSave

[mtrezza]

Issue Description

It is currently not possible to reference an object in its beforeSave hook to create a new object with a pointer to it.

Steps to reproduce

Example scenario:
A user in User class should have a user profile in UserProfile class associated with it. A user profile has a pointer to the user.

It is currently only possible to create the user profile in the user's afterSave hook. But immediately after saving the user, other API calls may look for the user profile which does not exist. It is not a solution to create a user profile if it does not exists, as that can cause race conditions in which multiple user profiles are created. It is also not a solution to set the pointer to the user as unique in the mongoDB collection which prevents multiple user profile from being created but also causes data loss, because the information to update the user profile with is lost after the failed call.

Expected Results

It should be possible to create a pointer to an object in its beforeSave hook.

Actual Outcome

Trying to save a user profile causes a timeout in path="/parse/batch".

Environment Setup

• Server

  • parse-server version: 2.8.4
  • Localhost or remote server: Heroku

• Database

  • MongoDB version: 3.6.7
  • Localhost or remote server: mLab

Logs/Trace

[flovilmart]

This is the expected behavior as the object is not stored in the DB yet and doesn’t have an objectid that is stored.

Why can’t you use the afterSave for this intent?

[mtrezza]

Because after saving the user other methods try to update the user profile. This can happen immediately after sign-up when the user is created. In some cases the afterSave hook has not created the user profile yet and the user profile cannot be updated. I tried to solve this by creating a user profile if it doesn’t exist when trying to update it (via cloud code). But that can also cause race conditions when the update cloud code is called multiple times immediately after each other.

[flovilmart]

ok; I believe I get the problem, however creating a pointer to an unexisting object is not allowed, and at the time of the save, the object is not saved (by definition). Anything could prevent the object from saving actually.

What do you suggest?

[acinader]

@mtrezza, you had the right idea asking this on StackOverflow: https://stackoverflow.com/questions/53321248/how-to-prevent-race-condition-when-creating-a-document-on-parse-server

this is a design issue that you can solve.

I'm glad to continue helping you on that thread.

[mtrezza]

@acinader, thank you, any help appreciated.

My understanding is that there is no solution to the issue. There is always a possible race consition. Hence the issue opened here to reflect on the parse server design.

What is your suggestion to solve this?

[acinader]

I asked you a question on SO: "so why even bother doing #2? why not count on #4 to do the work."

Danh and I are trying to help you, but I am currently lacking enough information to do that.

[mtrezza]

@acinader I already answered this on SO two days ago. Which information specifically do you need?

Again, there seems to be no way to avoid a race condition so I think it is not just a debugging issue.

[flovilmart]

While I appreciate all kinds of discussions, this one has nothing to do here @mtrezza =. double posting on multiple forums is never the way to go. Here, I particularly addressed the fact that what you are facing is indeed not a bug. If you wish to open a PR and suggest a 'safe' way to save pointers to unexisting objects feel free to do so. Otherwise, I would appreciate you stick with stack overflow.
Never you mentioned in your description you'd open a question outside. Not being satisfied with the answers on stackoverflow this is not a valid answer. And yes race condition there is. If you want to avoid races, I would recommend you use atomic updates, or another way to structure your data.. OR use another DB technology like Postgres that offer transactions.

[mtrezza]

@flovilmart Agreed. As the community confirms that parse server cannot do this by design, I will look into opening a PR. I just wanted to make sure that I did not miss a feature or even an existing PR that actually addresses this.

[acinader]

In this case, since you're not trying to coordinate multiple clients, I think that there are ways to work around the problem with parse server as is. But, there are definitely times when I could have used a 'blocking after-save hook' where the object is available in its saved state before the client gets a response. Glad to re-open @mtrezza if you want to work through it.

[flovilmart]

I’ll be closing again as stalled and against the basic protection that the pointer objects should exist in the DB.

[mtrezza]

OK, I will look into a PR when I have more time.

If anyone interested, the currently used workaround:

• a unique index in mongoDB on the user field in UserProfile collection
• a Parse.Cloud function that returns the user profile and creates one if none exists yet

To address concurrency: if no user profile was found and creating a user profile fails with Parse error 137 DuplicateValue, then the function queries again for an existing user profile.