I can't seem to find any information in the documentation or via Google on this, but if there is something, a pointer to it would be great.
In my app, I have a Thing as a core data class. I intend to have that Thing contain many Items which has a bunch of fields in it, like order and created_date and so forth. However, there are a variety of Item types, each with their own set of fields. Ideally, I'd like to create several subclasses of Item, so that I can access all the items together in a single array or something.
In Rails, I'd use STI for this. Does Core Data support similar behaviour?
Thanks!
You can create an Item abstract entity and then have each of your unique items extend from it. Keep the relationship in the abstract so that your Thing can see all of them.
Be warned, however, that under the hood, all of those children will actually be put into a single wide table so you will need to test for performance considerations.
Related
We develop the back office application with quite large Db.
It's not reasonable to load everything from DB to memory so when model's proprties are requested we read from DB (via EF)
But many of our UIs are just simple lists of entities with some (!) properties presented to the user.
For example, we just want to show Id, Title and Name.
And later when user select the item and want to perform some actions the whole object is needed. Now we have list of items stored in memory.
Some properties contain large textst, images or other data.
EF works with entities and reading a bunch of large objects degrades performance notably.
As far as I understand, the problem can be solved by creating lightweight entities and using them in appropriate context.
First.
I'm afraid that each view will make us create new LightweightEntity and we eventually will end with bloated object context.
Second. As the Model wraps EF we need to provide methods for various entities.
Third. ViewModels communicate and pass entities to each other.
So I'm stuck with all these considerations and need good architectural design advice.
Any ideas?
For images an large textst you may consider table splitting, which is commonly used to split a table in a lightweight entity and a "heavy" entity.
But I think what you call lightweight "entities" are data transfer objects (DTO's). These are not supplied by the context (so it won't get bloated) but by projection from entities, which is done in a repository or service.
For projection you can use AutoMapper, especially its newer feature that I describe here. This allows you to reduce the number of methods you need to provide "for various entities" (DTO's), because the type to project to can be given in a generic type parameter.
I hope someone can give me some guidance in how to best approach this situation.
I am using dbcontext, wpf and sql server.
I am having situations were the presentation of the data requires other data than just what is coming from a single table. For example, if I had a person table but wanted to show also how many books they had read from related data, say fields would be name, address, NoOfBooks.
I currently create a new class, called say PersonBookPM, that I fill up with data from a linq query which combines the two tables which includes the above three fields.I create an observablecollection of that and make that the itemssource of the grid/listbox.
When I am then adding data to that I then need to use the selecteditem, convert that back to the single entity of person, and attach it back in to the context.
It seems like the classes have already been defined by the code gen and I am repeating the process only slightly differently.
Am I going round the houses here?
Thanks Scott
I want to be able to look through all the attributes of an Entity and find the most popular one. I know it has something to do with NSPredicate, but I can't quite wrap my mind around to achieve it.
One possible solution:
Fetch all the entities and loop through it and sort the attributes into different arrays, from there count the items in the arrays to determine the most popular/common one.
Although this might work I'm just wondering if there's an easier or 'cleaner' way of doing it.
Update:
Thanks #Caleb. Let me clarify, I'm looking for a single attribute value that's most often used by instances of a given entity.
That is really a dirty descision.
I would suggest you to make a new entity, say, AttributeCounter, with two attributes - name and count, and every time you add an attribute to a person, change this entity.
But that would only be good descision if you have a few different attributes and lots of persons. If not, here is another approach, that is quite simple:
Get all the enteties with first attribute not nil,count,add to array
Sort it
Here you are
I have started developing my own web framework which has been coming along quite nicely. It simplifies and cators for all my specific needs. I have tried many frameworks like struts, struts 2, tapestry, spring and the list goes on. There are 2 big problems I find in all of these frameworks.
The ability to work with specific business logic/rules.
I am a bit of a perfectionist and the entire layout of these frameworks force you to scatter you validation, DAO logic and your form actions. I like to keep things grouped together and not do validation in my Hibernate Objects, Action classes and a separate XML files which causes a load unnecessary processing.
Ok back to my question. Does any one know of possible solutions to populating Objects from a HTTPRequest? I started looking into introspection and reflection, but I would like to see if there might be a better solution for something like this. Example: Object Student has 3 fields eg String Name, Integer age and Calendar DOB. What are the options in populating this object or any other Object from a HTTPrequest?
You could use a similar system to the way objects are sent in json by adding a __type__field that gets sent with each object. That way you know which object to create and introspect.
What is the best way to implement arbitrary row reordering in a tableview that uses core data? The approach that seems obvious to me is to add a rowOrder attribute of type Int16 to the entity that is being reordered in the tableview and manually iterate through the entity updating the rowOrder attributes of all the rows whenever the user finishes reordering.
That is an incredibly inelegant solution though. I'm hoping there is a better approach that doesn't require possibly hundreds of updates whenever the user reorders things.
If the ordering is something that the data model should modal and store, then the ordering should be part of the entity graph anyway.
A good, lightweight solution is to create an Order entity that has a one-to-one relationship to the actual entity being ordered. To make updating easy, create a linked-list like structure of the objects. Something like this:
Order{
order:int;
orderedObject<--(required,nullify)-->OrderObject.order
previous<--(optional,nullify)-->Order.next;
next<--(optional,nullify)-->Order.previous;
}
If you create a custom subclass, you can provide an insert method that inserts a new object in the chain and then sends a message down the next relationships and tells each object to increment its order by one then the message to its next. A delete method does the opposite. That makes the ordering integral to the model and nicely encapsulated. It's easy to make a base class for this so you can reuse it as needed.
The big advantage is that it only requires the small Order objects to be in alive in memory.
Edit:
Of course, you can extend this with another linked object to provide section information. Just relate that entity to the Order entity then provide the order number as the one in the section.
There is no better way and that is the accepted solution. Core Data does not have row ordering internally so you need to do it yourself. However it is really not a lot of code.