Can you explain a "transient attribute" in Ruby on Rails?
I'm reading the factory_girl Getting Started guide and its making reference to transient attributes.
[https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md#defining-factories][1]
I understand "attribute" as an instance variables. But "transient attribute" vs. "attribute"; not so sure.
I'm new to Ruby on Rails and this would be helpful.
"A transient attribute does not get persisted to the database. For instance, in one of my models, I have a transient attribute called last_updated_by. I don't save it to the database; I just use it during the processing of the model object by the controller."
Thanks J Plato!
Related
I've been plagued for months with an error in ODM.
It doesn't prevent my services from working, it only prevents ODM from automatically generating the JSON input payload when I test a service through the Rule Execution Server (the Retrieve HTDS Description File).
I attached a screenshot of the error.
What is puzzling is that:
if I move my variable from Input to Output, then the JSON gets generated in the Server Response section
if I manually enter my own JSON, the service executes correctly and I get back a decision
Does anyone have any idea what could possibly be wrong here?
Very likely your variable is not being deserialized properly. This would explain both the error you are getting and why you get it only as an input variable.
We get a similar error using a java.tim.ZonedDateTime with ODM 8.9.1, which does not support the java.time package. We just ignore the error -- and are hoping we can avoid it when we move to ODM 8.10.2.
hard to help as some of the context is missing. However the symptoms you describe tells there is a serialization issue one of the class of the HTDS interface.
It could be :
a getter/setter missing
a dependency missing in the XOM (classloader)
or a #jsonIgnore missing on a non getter/setter method whose name starts by get or set.
Best
Emmanuel
I have a core data database. I'm performing a custom migration. I have a subclass of NSEntityMigrationPolicy. My policy's migration method buildValueFromSectionFieldManagedObject: is being called by a mapping rule : FUNCTION($entityPolicy, "buildValueFromSectionFieldManagedObject:" , $source).
This part is actually working.
However, the implementation of buildValueFromSectionFieldManagedObject: uses methods in the custom entity NSManagedObject subclass of the $source, which is Choice.
The methods of Choice do not seem to be available to the migration function, and instead it gets just a vanilla NSManagedObject.
When I try to use Choice methods, I get an exception. If I po the choice in the debugger, I get something like this:
<NSManagedObject: 0x600000281860> (entity: Choice; id: 0xd00000000038001a ; data: )
Whereas, out of a migration I would usually see something like this:
<Choice: 0x60800028bdb0> (entity: Choice; id: 0x6080002225a0 ; data: {
Is this just how it is, or is there some way that I can use the entity objects during migration?
Possibly relevant – this particular entity, Choice, is removed during this migration. It does not exist in the target managed object model, but does exist in the source managed object model. However, I don't think this is the case as other entity classes that are in the target model are also unavailable as that class during migration – they have class NSManagedObject and their entity methods are not available.
That is correct, you only have access to basic NSManagedObjects during migration.
Three-Stage Migration
The migration process itself is in three stages. It uses a copy of the source and destination models in which the validation rules are disabled and the class of all entities is changed to NSManagedObject.
From: Core Data Model Versioning and Data Migration Guide
Dave's answer clarified that during a migration, core data object are only available as NSManagedObject instances. You don't get to use their Entity classes.
Worse, if you're using a tool like mogenerator, then any handy logic that you've extended the entity classes with is inaccessible.
Poor solutions
Working with an NSManagedObject directly feels dangerous to me. Using [managedObject valueForKey:#"someKey"] is verbose, but worse, there's no compiler checking that you've got your key name correct, so you might be asking for something managedObject doesn't have. There's also no compiler checking of the returned type either – it could be anything that you can put in to a managed object.
Slightly better is [managedObject valueForKey: NSStringFromSelector(#selector(someKey))] is safer, but horribly verbose and awkward, and still isn't that safe – lots of things might implement the method someKey.
You might also declare your keys as literals:
NSString *const someKey = #"someKey";
[managedObject valueForKey: someKey];
Again – this is slightly safer, less error prone and verbose, but you've got no guarantee still that this managed object has someKey.
None of these approaches will give us access to custom logic, either.
Better solution
What I did instead of this was to define a protocol for the properties that I know my managed object has. Here's are examples for entities Choice and ChoiceType.
#protocol ChoiceProtocol
- (NSSet<id <ChoiceTypeProtocol> > *)choiceTypes;
- (NSNumber *)selected;
- (NSNumber *)order;
#end
#protocol ChoiceTypeProtocol
- (NSNumber *)selected;
- (NSString *)name;
- (NSString *)textCustom;
- (NSNumber *)order;
#end
Now in my migration code, instead of having a custom migration function similar to:
- (NSString *)migratedRepresentationOfChoice:(NSManagedObject *)choice;
I have:
- (NSString *)migratedRepresentationOfChoice:(id <ChoiceProtocol>)choice;
In the body of this function I can use choice exactly as I would any regular object. I get code completion as I type, I get the right syntax highlighting, the compiler will complain if I call a non existent method.
I also get return type checking, so the compiler will complain if I use the NSNumber property selected as an NSString or NSSet. And it'll also be helpful and suggest NSNumber methods as completions while I type.
How about the logic?
This approach doesn't provide the logic you've added to entity classes.
In Swift you might be able to use a protocol extension to achieve that.
I was retiring these entities (hence the migration), so I moved the entity logic functions I needed in to a helper functions of my custom migration. As an example choice.orderedChoiceTypes becomes [self choiceOrderedChoiceTypes:choice].
In future I will probably avoid ever adding logic to NSManagedObject entities. I think it's probably a better plan to put any such logic in domain objects that you build from your managed objects. Further, I will probably avoid defining entity classes and instead only access NSManagedObject instances through a protocol as during the migration. It seems clean, simple, removes magic, and has benefits for testing – not just for migrations.
I know it seems a stupid question, but after modelling the typical master-detail or parent-children relationship in Core Data, I want to create the NSManagedObject subclasses and I realised Core Data code generator is always using strong/retain for the parent properties. Is that right? In a normal object graph that is wrong because we have a retain cycle, but not sure in the Core Data context, because I guess the code generator should generate the code properly. Thanks for suggestions.
After trying to use weak properties in my NSManagedObject subclasses, I receive:
2016-03-15 11:57:36.921 WWWWW[84639:1153803] CoreData: error: Property 'company' is marked __weak on class 'Package'. Cannot generate a setter method for it.
2016-03-15 11:57:36.921 WWWW[84639:1153803] -[Package setCompany:]: unrecognized selector sent to instance 0x7f97917068e0
2016-03-15 11:57:36.933 WWWW[84639:1153803] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Package setCompany:]: unrecognized selector sent to instance 0x7f97917068e0'
I have a Core Data "List" entity that has an ordered relationship to ListItems.
Core Data itself, generates a method in the .h,
- (void)removeListItemsAtIndexes:(NSIndexSet *)indexes;
However, when I attempt to invoke that method, at runtime the app crashes stating its an unrecognized selector.
Is this method really not implemented? Why would Core Data declare it if its not implemented? Am I supposed to do something else to make this work?
Thanks
You should look at this post:
problems-with-nsorderedset
it's lion app, but i think the problem is same.
I have a question that keeps bothering me. Currently, I have started using Kohana 3.2 Framework. I've written a helper to handle some functionality - I have a number of methods, which are (as it should be) declared STATIC. But, all of these methods are somehow working with the database, so I need to load a model. Currently, every method has a non-static variable like this:
$comment = new Model_Comments;
$comment->addComment("abc");
OK, it seems to be working, but then I wanted to get rid of this redundancy by using class attribute to hold the instance of the model (with is class as well).
Something like this:
private static $comment; // Declaring attribute
self::$comment = new Model_Comment; // This is done within helper __constuct method
self::$comment->addComment("abc"); // And call it within the method.
But, I got failed with: Call to a member function addComment() on a non-object
Question is: is it possible to do it ? Maybe there are some other approaches ?
Sorry for a long story and, thanks in advice! :P
A static method cannot call a non-static method without operating on an instance of the class. So, what you're proposing won't work. There may be a way do accomplish something similar, but what about trying the following:
You could implement the singleton or factory pattern for your "helper" class. Then, you could create the model (as an attribute) as you instantiate/return the instance. With an actual instance of your "helper" class, you won't have to worry about the static scope issues.
In other words, you can create a helper-like class as a "normal" class in your application that, upon creation, always has the necessary model available.
I'd be happy to help further if this approach makes sense.
David