I am new to Golang. I have this example here - https://go.dev/play/p/lusSZk5be4b
I am trying to update the global parent structure from one of the elements of the same parent structure. I was expecting this program might create an issue because of updating the parent structure from it's own child but this seems to work fine.
I did not understand if this is the right behaviour and is accepted in golang or this kind of updating the parent from child should not be done at all. Any help is appreciated.
Thank you.
You're not actually replacing the parent, but the global variable.
If you keep a reference to the original parent you'll see things remain the same there:
https://go.dev/play/p/FsNVdheZPfE
func main() {
p = &parent{}
oldParent = p
...
fmt.Pritnln(oldParent)
...
Even if you had an actual parent on the child struct and update it (example: https://go.dev/play/p/hELKFB7DWc- ) , the original reference would still have the child, but the child would have a parent with no children.
So in summary, you're just updating references which is totally valid. Whether that makes sense in code or you might lose an important reference that's a different thing.
I have an NSManagedObject with an NSOrderedSet to-many relationship named subpointConnections_. I am observing changes to this set using the following:
addObserver(self, forKeyPath: #keyPath(subpointConnections_), options: [], context: &subpointsOC)
This can be called only once in the initialisation of a lazy property on the NSManagedObject.
Any new relations are added in a child NSManagedObjectContext, and then saved down to my main context.
When I set the relationship, I use the following code, only setting the 'many' side of the relationship from the 'one':
superpoint.insertIntoSubpointConnections_(self, at: index)
insertIntoSubpointConnections_ is an autogenerated accessor.
In the child NSManagedObjectContext I see a single correct call to observeValue(forKeyPath..., that gives me the correct insertion index on subpointConnections_, when the above insertion code is called.
However, when I save to the parent NSManagedObjectContext I am seeing two calls to observeValue(forKeyPath... on the same object. The first call is for an insertion at the end of the NSOrderedSet. The second call is for an insertion at the correct position.
When evaluating the NSOrderedSet in each observation call, the new object is indeed at the position described - it is on the end of the set on the first call, and then moved to the correct position on the second call. However, I get no call to say that it was removed from the end position.
I am not sure if I have set something up incorrectly here. The many objects in my one-many relationship will be unique, so I could write the code to work around this, but that feels wrong.
Any thoughts on what I might be doing wrong?
Weirdly, the first change dictionary passed to observeValue(forKeyPath... contains an NSIndexSet, the second contains an NSMutableIndexSet.
EDIT
I have replicated this issue in a simple project here: https://github.com/GilesHammond/KVO-Core-Data-Extra/
Run in debug and select "ADD CHILD" multiple times from the app Main Menu. Observe the debug output showing the extra erroneous observation on the main NSManagedObjectContext.
I contacted Apple Support and have submitted this issue as a bug as suggested, as the given notifications are not atomic.
I had to take a different approach. I now update my NSManageObject when the NSManagedObjectContextDidSave notification is posted.
Suppose I have the following nested UI-Router route:
/parent/{parentId}/child/{childId}
parent:child in my case is a one:many relation, therefore any valid childId implies a specific parentId. I'd like to maintain the nested state in my application, keeping access to parent resolve dependencies without reloading between sibling children; but instead represent the above with the terser URL:
/child/{childId}
Ideally, I'd like UI-Router to do as much of the lifting as possible, and only write the action to recover just the parent state when it is lost (for example when loading the entire page from a child route URL).
My responsibility could, for instance, be simply handling recovery of the parentId state parameter when it is null.
Is this feasible?
edits day 2: It seems, according to the docs, that UI-Router is designed to allow obscuring parent routes from the URL using Absolute Routes. I can only assume that it is supposed to preserve the parent state normally in this case. So, I tried a quick implementation by including two identical URLs, one an absolute URL that is a child state of the parent, and the other that is an actual root state, which does nothing but resolve the parent parameter, then load a controller to perform a redirect to the actual parent/child route. It had lots of problems. I'm trying to determine if I can do the same thing with an abstract state above the parent route. Anyway, suffice to say I haven't solved this yet.
I have a simple 1:1 relationship:
class MyParentDomain{
String name
MyChildDomain onlyChild
}
class MyChildDomain{
String name
}
Now on a form where I want to make a new parent "Mom", there will be a list with all the existing children. Is there a current good-practice to add children on that same form? I'm imagining a "plus" button next to the drop-down list of "onlyChild" where I could see a form for new "MyChildDomain". I saw a link somewhere where people were talking about cracking this problem (will add the link as soon as I re-discover it). Has this been done? Is anyone doing something like this?
To my knowledge, this hasn't been generally applied to any templates or with a plugin (I might be wrong).
You can, however, use the "list" abilities that Grails has which allow you to submit a list of domain entity data and then build the list of child elements in the controller from the submitted data and persist that. I've never done this myself, however, so you will need to do a bit of digging on it.
As you may have guessed, attempting to create child entities on the fly via Ajax is likely not to work since the parent entity doesn't exist yet, so there's nothing to attach them to.
I'm fairly new to the OO design process, so please bear with me....
I have two entities that I need to model as classes, call them Parent and Child (it's close enough to the actual problem domain). One Parent will have one or more Children -- I have not interest, in this application, in childless Parents.
Where my brain is going out to lunch is on the fact that I need to be able to find either from the other. In my database I can implement this with a normal foreign key relationship, and the set-based nature of SQL makes it easy to find all Children for a given Parent, or the Parent for a given Child. But as objects...?
I think that the Parent should carry a collection (list, whatever) of Children. I also think that each Child should carry a reference to its Parent. The circular nature of the references, however, is making my head hurt.
Am I:
On the right track?
Completely off base? If so, what should I do differently?
This will almost certainly be implemented in VB.NET, but I'm a ways from cutting code yet.
Edit after 8 answers:
Thanks all. It was hard to pick just one answer to accept.
To clarify a couple of things that were questioned in the answers:
Parent and Child are very different
entities--there's not inheritance
relationship at all. I chose the
names that I did because they're
really very close to the real-world
problem domain, and now see that it's
a source of confusion from an OO
perspective.
The hierarchy is only one level deep--Children will never have Children
within the application.
Thanks again.
The circular references are fine and absolutely standard when creating a tree structure. HTML's Document Object Model (DOM), for example, has the parent and child properties on every node in a DOM tree:
interface Node {
// ...
readonly attribute Node parentNode;
readonly attribute NodeList childNodes;
// ...
}
Sounds like you're on the right track to me. As per your domain model, parents have children and children have parents. You may need to reference each from the other.
There is nothing wrong with circular references, you just have to be careful about what you do with them. Where you'll run into trouble is managing your entities on the server side in an automated fashion when you load them from the database. For example, you fetch a Child object from the database with a query. Do you include the parent information? Do you include the parent's children?
ORM tools like Lightspeed or Microsoft's Entity Framework generally deal with this using "lazy loading" directives. They'll fetch what you need at first (so, when you fetch a Child, it just gets the Child properties and the parent's ID). If later, you dereference the Parent, it goes and fetches the Parent properties and instantiates the Parent object. If later still, you access it's Children collection, it then goes and fetches the relevant child information and creates Child objects for that collection. Until you need them though, it doesn't populate it.
I think it's reasonable to want to be able to traverse the object graph in this way. It's hard to know if you have a justifiable reason for it from your post, but I don't think the references in and of themselves prove a bad design.
I believe you're on the right track. Why is the circular nature of the references making your head hurt? What is the fundamental issue you're having with a Parent having references to its children, and a Child having a reference to its parent?
Are you talking about a class hierarchy, where the parent class knows about its child classes?
You should avoid this at all costs.
By default, a child class knows all about a parent class, because it is an instance of the parent class. But to have a parent class know about its child classes requires that the child class also know all about every other child class. This creates a dependency between one child and every other child of that class. This is an unmaintainable scenario that will cause problems in the future -- if you can even get it to compile or run, which in many languages will not be the case.
That said, it sounds to me like you're not trying to do a class hierarchy, but a collection hierarchy, i.e. a tree. In that case, yes, you're on the right track; it's a common paradigm. The parent node has a collection of child nodes, and the child node has a reference to the parent node.
The thing is? They're all the same class! Here's a very simple example in C#:
public class Node
{
public readonly Node Parent; // null Parent indicates root node
public readonly List<Node> Children = new List<Node>();
public Node(Node parent)
{
Parent = parent;
}
public Node()
{
parent = null;
}
public void AddChild(Node node)
{
Children.Add(node);
}
}
I have a feeling this is what you're really after. Using this paradigm, you would then sub-class Node for whatever nefarious purposes you might have.
If I understand that objects of P contain an array of objects P->c[] representing children. And any node P with no children is a leaf ... with each P containing P->P' (the parent).
The solution you specify, with Parents containing references to children and vice versa eliminates the need to traverse the tree to obtain ancestry of a given child and children of a node. This is really just a tree that can you perform all kinds of links on and algorithms to traverse and enumerate it. Which is fine!
I suggest reading the trees chapter in The Art of Computer Programming for an excellent and in-depth look at tree structures and efficient ways to enumerate parentage and children.
If the children must have a parent I usually just require a parent type instance in the child constructor.
Sounds to me like you're on the path to a bad design. Your architecture should never have circular references.
You should probably re-examine why your children need a reference back to the parent and vice versa. I would lean toward the parent having a collection of children. You can then add functionality to the parent to check to see if a child object is a child of the instance.
A better explination of the goal might be a little more helpful as well...
EDIT
I read up a little more (and listened to comments)...and it turns out I'm quite in the wrong. Circular references do in fact have their place as long as you're careful with them and don't let them get out of hand.