Should I define the primary key for each entity in Realm?

Question

I have noticed that setting PK is not obligatory in Realm and simply can be omitted. But in documentation is stated that:

Indexes are created automatical

Answer 1

The horse has been beaten to death already, but I couldn't help but reference the Realm code which throws an exception if a Realm Object is created or updated without having a primary key.

+ (instancetype)createOrUpdateInRealm:(RLMRealm *)realm withValue:(id)value {
    // verify primary key
    RLMObjectSchema *schema = [self sharedSchema];
    if (!schema.primaryKeyProperty) {
        NSString *reason = [NSString stringWithFormat:@"'%@' does not have a primary key and can not be updated", schema.className];
        @throw [NSException exceptionWithName:@"RLMExecption" reason:reason userInfo:nil];
    }
    return (RLMObject *)RLMCreateObjectInRealmWithValue(realm, [self className], value, true);
}