Update CoreData entity obj - objective-c

Question

I have a complex CoreData entity: MY_ENTITY
I receive a Object of type MY_ENTITY from my webService.
In some cases, I need to edit my local CoreData obj (MY_ENTITY) with received obj.
So:
I have OBJ_1 in CoreData
I receive OBJ_2 from WebService.
I need to update OBJ_1 from OBJ_2.
Have I to set all field or can I assign OBJ_1 ObjectID to OBJ_2 and save the context (same Context)?

Answer 1

Since they are two separate instances, you need to move what you want from O2 to O1. You can use a routine such as this to do the move attribute by attribute assuming both objects are of the same Entity class:
// use entity description to get entity attributes and use as keys to get value
// scan attributes
NSDictionary *attributes = [[sourceEntity entity] attributesByName];
for (NSString *attribute in attributes) {
id value = [sourceEntity objectForKey:attribute];
if (value == nil) {
continue;
}
NSAttributeType attributeType = [[attributes objectForKey:attribute] attributeType];
switch (attributeType) {
case NSStringAttributeType:
// value = [value stringValue];
break;
case NSInteger16AttributeType:
case NSInteger32AttributeType:
case NSInteger64AttributeType:
case NSBooleanAttributeType:
value = [NSNumber numberWithInteger:[value integerValue]];
break;
case NSFloatAttributeType:
case NSDecimalAttributeType:
value = [NSNumber numberWithDouble:[value doubleValue]];
break;
case NSDateAttributeType:
if (dateFormatter != nil)
value = [dateFormatter stringFromDate:value];
break;
default:
value = #"";
break;
}
[targetEntity setValue:value forKey:attribute];
}
Note, this is just an example and would need to be cleaned up and have error handling added if you intend to use. Also, if you are getting O2 via a webservice as JSON or XML, then you can use this to simply push the JSON payload into the targetEntity. This assumes your payload attrs align with your entity attrs. In this case you would replace sourceEntity with the JSON payload using a block or equivalent for example:
NSArray *seedData = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath]
options:kNilOptions
error:&err];
[seedData enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
...
id value = [obj objectForKey:attribute];
...
}

Answer 2

In which format are you receiving OBJ_2 from the Web Service?
Either way, assigning OBJ_2 to OBJ_1 would not work since you would only replace the reference your local variable is pointing to.
To synchronize your local CoreData Entity which the data from the server you will need to modify the attributes of your existing entitiy. Depending on your data model and the format you are receiving OBJ_2 in there are different ways to achieve this.