EdgeQL help is needed. "Link filtering" question.

[scorday]

Hello. I understand that my question is kinda basic, but I could not find any help in older discussions.

I have this schema:

`
type User {
}

type Project{
private_user: User {
constraint exclusive;
};
}
`
Projects can have and have not private_user link. How can I select projects that has .private_user = {} (unset) and .private_user != user1 ?

I tried:

1. select Project { * } filter .private_user != (select User filter User.id = <uuid>$arg limit 1) or not exists .private_user;
2. select Project { * } filter not exists .private_user or .private_user.id != <uuid>$arg;
3. select Project { * } filter .private_user = <User>{};

I have project with no private_user in db, but every time i get empty results. What's wrong?

[vpetrovykh]

Right, so what you want is to use coalescing comparison.
There are ?= and ?!= that do the kind of thing that you want:

Your expression (1) could be re-written like so:

select Project { * } filter .private_user.id ?!= <uuid>$arg;

Which will get you all the projects that don't match the argument id as their private_user.id. And an empty set id will also not match any id you pass, thus all projects without a private_user will also be returned by this query.

The reason why this is happening is that most basic operators when dealing with sets are applied to the cross-product of those sets (so all possible pair combinations). So select {0, 1, 2} + {10, 20} will get you {10, 20, 11, 21, 12, 22}. But that also means that select {} + {10, 20} gets you {}. The same applies to =, != and even and and or. When comparing potentially empty sets you should use ?= and ?!= instead, if you want the {} be treated like a valid comparison value. To combine {} with other operators of functions you might need ??, which lets you provide a default value to be used instead of the {}. This is because a missing value doesn't always have a natural interpretation in most cases, so the way for you to provide it is via the coalescing operator ??:

_localdev:main> with a := <int64>{} select (a ?? -100_000) < 23;
{true}
_localdev:main> with a := <int64>{} select (a ?? 100_000) < 23;
{false}

[scorday]

Thanks!!!