On Mon, May 29, 2017 at 6:40 AM Darko Luketic <i...@icod.de> wrote:
>
> when I use a concrete model.User (aka line 20) everything works without
> error.
> but when I use var m storage.ModelInterface and then call a member
> function of the interface (SetName)
> Go panics because nil pointer dereference
>
> === RUN TestNewUserService
> --- FAIL: TestNewUserService (0.00s)
> panic: runtime error: invalid memory address or nil pointer dereference
> [recovered]
> panic: runtime error: invalid memory address or nil pointer
> dereference
> [signal SIGSEGV: segmentation violation code=0x1 addr=0x80 pc=0x5ba9f8]
>
> goroutine 5 [running]:
> testing.tRunner.func1(0xc420068b60)
> /usr/lib/go/src/testing/testing.go:622 +0x29d
> panic(0x5f1520, 0x721990)
> /usr/lib/go/src/runtime/panic.go:489 +0x2cf
>
> git.icod.de/dalu/forum/server/service.(*UserService).CreateUser(0xc420055f40,
> 0x62f170, 0x8, 0x630677, 0xd, 0x62e844, 0x6, 0x7ffdb1d66ba1, 0xc420031f68)
> /home/darko/go/src/
> git.icod.de/dalu/forum/server/service/user.go:19 +0x28
> git.icod.de/dalu/forum/server/service.TestNewUserService(0xc420068b60)
> /home/darko/go/src/
> git.icod.de/dalu/forum/server/service/user_test.go:23 +0x1f9
> testing.tRunner(0xc420068b60, 0x63af98)
> /usr/lib/go/src/testing/testing.go:657 +0x96
> created by testing.(*T).Run
> /usr/lib/go/src/testing/testing.go:697 +0x2ca
> exit status 2
> FAIL git.icod.de/dalu/forum/server/service 0.005s
>
> However I don't know what the problem was that I had previously an
> interface *does have* allocated memory but not in the same way the concrete
> model that fits the interface does so they're not compatible, which lead me
> to some headaches.
> So when an interface has memory allocated, why can't I use it as if it has
> memory allocated?
>
The zero value for an interface is nil. It has no concrete implementation
on its own. So what concrete logic would you expect a nil interface to
call? If you are trying to have per-database specific concrete model
implementations then something has to provide that concrete implementation
to your function. How are you intending the database implementation to be
selected in this application? Your service accepts a storage interface but
not a model interface. So what provides the model?
>
> As it is right now I'll have to dump all the StorageInterface and
> ModelInterface interfaces because it doesn't make sense to use them since
> I'm bound to a specific model/db at the topmost level,which is "service".
> "And this is why we can't have nice things".
> Removing those would also lower the memory footprint when querying for
> multiple results (slices, find/readall).
> It's sad, I really wanted to, but I can't.
>
> So lesson learned:
> - keep models the way they are, conrete for the expected database. A
> mongodb model is different from a pgsql model is different from a cassandra
> model
> - storage/repository can be concrete and it should just do CRUD
> - interface only at the top level, aka service
> - there is no 1 fits all in Go
>
> routes <> handler <interface> service <concrete> storage/repository
> <concrete> model
>
> effectively
> MySQLUserService
> MongoDBUserService
> CassandraUserService
> RethinkDBUserService
> but
> type UserService interface {
> //...
> }
>
> Question, is there any sane way to solve it without throwing it all away?
> Hope dies last. Any trick to make it still work with interfaces on the
> service level without needing to create a concrete database bound instance
> of a model/struct?
>
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