Caused by: java.lang.IllegalArgumentException: Invalid query: field '_id' not found in class 'cards'.
   at io.realm.internal.fields.CachedFieldDescriptor.compileFieldDescription(CachedFieldDescriptor.java:80)
   at io.realm.internal.fields.FieldDescriptor.compileIfNecessary(FieldDescriptor.java:292)
   at io.realm.internal.fields.FieldDescriptor.getColumnIndices(FieldDescriptor.java:185)
   at io.realm.RealmQuery.equalToWithoutThreadValidation(RealmQuery.java:309)
   at io.realm.RealmQuery.in(RealmQuery.java:551)
   at io.realm.RealmQuery.in(RealmQuery.java:531)
   ...

@RealmClass(name = "cards")
public class CardModel implements RealmModel {

    @PrimaryKey
    @Required
    @RealmField(name = "_id")
    private String mKey;

    // getters and setters
}

...

try (final Realm realm = mRealmProvider.get()) {
	final RealmResults<CardModel> all1 = realm.where(CardModel.class)
			.in("mKey", map(keys)) // will NOT crash here
			.findAll();
	final RealmResults<CardModel> all2 = realm.where(CardModel.class)
			.in("_id", map(keys)) // will crash here
			.findAll();
}