Disabling networking stops emitting data from realm - android

I am facing a problem with Retrofit and Realm with RxJava:
I have a realm observable using Observable.just(realm.copyFromRealm(realm.where(FooBar.class).findAll())) and an retrofit observable using mFooBarService.getFooBars().
I want to combine those so that first the database (realm) and then the api (retrofit) emits the data. I do this by concating them: Observable.concat(realmObservable, apiObservable)
This works good. I can also stop my api server, everything is working fine because the database has the data. Now the problem: if I turn off networking on my mobile phone (wifi, mobile data) no data is received by the observer. I thought that the observer first receives the database data and then a timeout or connection error? Instead I only get an error.
Thank you in advance!
Edit:
Ok, I try to clarify what I mean.
It looks like this if I have network connection but the server is down:
realmObservable.concat(apiObservable)
-----A---B---C---D---|---ERROR-------->
So I get the data and then an error
But when the server is down it looks like this above too but instead I only get an error and not the data first.
I hope you understand me now.

You are getting onError call from Retrofit and not from Realm. Your design of concatenation of Realm + Retrofit seems fine, what you are missing here is error handling for Retrofit when there is no internet connection. You can verify this by commenting all Realm code, and still receiving Error.
There are 2 ways to fix this,
Check for internet connection availability before making retrofit
call
Add custom error handling in retrofit

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