更新领域表

Question

I was just wondering if what I'm doing to update a entire table in realm is the correct safe approach. I receive a list of conversations from the server and update my db like this:

@Override
public void saveConversations(final List<Conversation> conversations) {
    realm.executeTransactionAsync(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) {

            // Remove all conversations and replace with the one passed in
            // Realm does not support cascade deletes. Remove messages as well
            // https://github.com/realm/realm-java/issues/1104
            realm.delete(Conversation.class);
            realm.delete(Message.class);

            realm.copyToRealmOrUpdate(conversations);
        }
    });
}

Conversation.java has a RealmList of messages inside:

public class Conversation extends RealmObject {

    private RealmList<Message> messages = new RealmList<>();

This works, I couldn't find any bugs with it but it does not look particularly elegant. What if realm.copyToRealmOrUpdate(conversations); goes wrong? I would lose all my data.

Anyways, I know this is not very probable, but I was wondering if there was a better way of doing things.

PS: bear in mind that I delete everything from the db because I don't want conversations in my db that don't exist in the server anymore.

Answer 1

Don't forget that you're executing a transaction, so if copyToRealmOrUpdate() fails, then the transaction is cancelled , which means you wouldn't lose all your data.

Personally, I used to go with the "delete all" approach, and if you can clear out all tables then it won't cause issues, but if you have a third table where you're referring to Conversation and Message (for example User.class ), you'd be invalidating all relations. So I personally prefer to merge like this.

• merging data and removing all data that's not in the list you've saving

.

public class Contact {
    @PrimaryKey
    private long id;

    @Index
    private String name;

    @Index
    private String phoneNumber;

    @Index
    private boolean isBeingSaved; // this line is important

    //getters, setters
}

Then merge:

// background thread
Realm realm = null;
try {
    realm = Realm.getDefaultInstance();
    final List<Contact> contacts = getContacts();
    realm.executeTransaction(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) {
            realm.insertOrUpdate(contacts);
            realm.where(Contact.class)
                 .equalTo(ContactFields.IS_BEING_SAVED, false) // compile 'dk.ilios:realmfieldnameshelper:1.1.0'
                 .findAll()
                 .deleteAllFromRealm(); // delete all non-saved data
                        // in your case, this is where you'd handle the cascading too though manually
            for(Contact realmContact : realm.where(Contact.class).findAll()) { // realm 0.89.0+
                realmContact.setIsBeingSaved(false); // reset all save state
            }
        }
    });
} finally {
    if(realm != null) {
        realm.close();
    }
}