getAllContacts.kt

fun getAllContacts(): Single<Map<Long, ContactModel>> {
    return Single.fromCallable { loadAllContacts() } // here we load all contacts
        .map { addPhoneNumbers(it) } // here we add phone numbers
        .map { it.filter { !it.value.phoneNumbers.isNullOrEmpty() } } // here we filter empty contacts
        .map { it.values.toList().groupBy { it.contactId }.mapValues { mergePhoneNumbers(it.value) } } //here additional merging happens
}