"Nullsafety" of Kotlin Room "Single element" Query...

# room

[Roar Gronmo]

"Nullsafety" of Kotlin Room "Single element" Query

Given Room Dao Query:

@Query("SELECT * FROM stations WHERE stationId = :stationId LIMIT 1")
    fun getStation(stationId: String): Station

What will happen if ":stationID" is not found, how is that element returned, null ?

Above expression is set as "nullsafe" (No warning telling that I should return Station? instead,) and handle the null check in my code.

AS should give a Lint Error/Warning here to tell that the result may return null.

My app fails at this point when handling a null (that obviously shouldn't be null...)

RG

[Roar Gronmo]

I made an issuetracker on this, and we'll see if its catched:
https://issuetracker.google.com/148596201

[kyleg]

Honestly I dont’ think it’s a linting issue. It’s an issue with the Room code that compiles an _Impl from the source interface. If the interface is written in Kotlin, the generated Java _Impl ought to do a null check and throw an exception.

As it stands, it’s up to you to do it, sadly.

[Roar Gronmo]

If there is an possibility of an API (as room is) returning null, Lint should flag that. I think this complies with the nullsafety of Kotlin.

[kyleg]

How should that work? A dao’s _impl is autogenerated Java, and Java has no nullable marker, so your linter cannot possibly know that the Java function could return null.

[kyleg]

Unfortunately, you have to outsmart the generator: any time your Kotlin DAO interface code SELECTs from the table, you should always put a ? at the end of your return value if you’re querying a single value.

Don’t do:

@Query("SELECT * FROM foo LIMIT 1")
fun getFoo(): Foo

Do:

@Query("SELECT * FROM foo LIMIT 1")
fun getFoo(): Foo?

or

@Query("SELECT * FROM foo LIMIT 1")
fun getFoo(): LiveData<Foo>

(the latter bakes in a warning about nullability since LiveData<A>.value: A? by definition

[Roar Gronmo]

Actually I agree with you here, but when inexperienced (and stressedout) programmers, there should just be a warning that the result may be null.

[kyleg]

The thing is, I don’t know what the fix is. If you just had a Kotlin linter that warned about every nullable return value coming from Java, since Java doesn’t have a nullable marker (aside from an optional @NotNull/@Nullable annotation), your linter would probably highlight almost every Java function you call since most of them do not have those annotations (since they’re optional).

[kyleg]

Room should have this prominently in its documentation about how to write a Dao interface

[kyleg]

IMO Room should still be labeled beta, because this is a preetty big issue. Another big issue is:

@Query("SELECT * FROM Foo")
fun getFoo(): Foo?

The query might select 50 rows, and it will pick 1 to return back. A query missing a LIMIT should error out on compile if your return value is not List<A>

[Roar Gronmo]

You coul add it as an option: Compile nullsafe -> This forces you to check for nulls on java libraries, Compile nullwarning ->You are wrned, Compile null silent -> You get no warning.

[kyleg]

I suppose so. We’re now venturing beyond my knowledge, as I have never messed with the Kotlin linter and my compile options :)

[Roar Gronmo]

Anyhow, kotlin is somehow ventured as a "nullsafe" language (opposite to java, c/c++ etc), so I think it would be acceptable to be more strict about it.