I have two entities Company and Employees. Company has a 1-to-many relationship with employees. An almost exact question was asked here however it doesnt answer my question :(. Like in that question, xcode in Company.m generates 4 almost exactly the same methods. However, the prototypes are not included in Company.h which leads me to believe that this is either
An error and that company.h requires the 4 prototypes for the void functions described in.
Intended and those 4 methods are only usable in the .m file? If so how to add many employees to a company.
Standing by for any clarifications. Thanks!
Typically I just set the company on the employee when creating the employees.
[newEmployee setValue:company forValue:#"company"];
this will trigger the relationship both ways and add the employee to the companies set of employees.
or since you've subclassed it.
newEmployee.company = company;
Related
I'm very new to Core Data and I have two questions:
I want that all of my Patient entities have property bed which is a value of type bed... Can I create an entity for that too and connect always one patient with one bed?
I wondered if I can do something like every Patient has a property doctors, and that would be a array of doctors "[doctor]". How could I make this.
Thanks, hope you know what I mean, it would be great If someone knows how to do that.
Okay Larme and Joakim Danielson answers my question as comments under my post :)
Their comments:
Yes. The key word you are missing is "relations". You want relationship between your entities. Be it one to one, one to many, many to many. –
Find an online tutorial on learning and using Core Data, many of them handles entity relationships as well –
Here's what I'd like to know.
Assumptions:
It's the year 2020, and not 2010
One database containing all relevant entities
One web API
Using Entity Framework
The question:
Is it typically considered best practice to break EF entities into...
A separate DbContext (and Repository) for each entity?
One DbContext to rule them all?
Something in between?
Is performance, maintenance and/or unit testing affected one way or the other? This being, practically speaking, my first rodeo, I'm unsure of the ramifications of my choice and would appreciate input from others who've been further down this road.
I'm asking this question today because I've got a dozen lookup tables whose data will rarely be edited. It's starting to feel like overkill to have a separate DbContext for each.
I could just throw those entities under the same context used by the table they supply look-ups to.
Or I could do even more, and pull a dozen related entities under the umbrella of a single DbContext.
Or I could just stuff literally the entire database into one DbContext.
And I cannot tell if there is a right or wrong answer or even if it matters in practice.
I found a number of related questions, but they were either asked a long time ago or weren't the same question. (In particular, I am not asking about what scope to use, as this answer is, I believe, a good one.)
I've included below some relevant links that I found thus far.
when-should-i-create-a-new-dbcontext
linq-and-datacontext
use-multiple-dbcontext-in-one-application
how-many-dbcontexts-should-i-have
how-many-dbcontext-subclasses-should-i-have-in-relation-to-my-models
I know this is an opinionated question. However it comes up often at work.
When creating methods it's often a struggle to know which class should be responsible.
e.g.
bool result = ProductService.CategoryHasSoldOutOfProducts(int categoryId)
vs
bool result = CategoryService.CategoryHasSoldOutOfProducts(int categoryId)
In my opinion, the CategoryService should be responsible, as the method is taking a categoryId and is specific to the Category.
Others at my work say the ProductService should be responsible as the method is dealing with if Products have sold out.
Just trying to develop a better understanding of service architecture and good process. I'm interested in other peoples explanations for why they would choose one over the other.
Thanks
Disclaimer - this is a purely IMHO answer. I am answering this in the spirit of having a design brainstorm.
Based on the OP, it seems the relationship between Category and Product is an optional one to many : Category (0..1) <--------> (*) Product.
Implementation wise, this means that the Category entity probably has a Container of Products, and the Product entity has a reference to a Category which may be NULL.
In this case, I agree with the decision to place CategoryHasSoldOutOfProducts under the responsibility of the Category entity. The method name clearly implies that the Category entity should be responsible for informing its API user on the status of its products.
There is another option, however: An association class/entity. The motivation behind this entity is to describe the relationship between two other entities.
In this case, you can have a functional association entity which we will call ProductContainment for the sake of this example.
ProductContainment will have no internal state, and will hold functions which are provided with Category and/or Product entities as parameters.
It is then the responsibility of the association entity to provide the implementation of functions which relate to how Category and Product relate to one another.
If you end up using ProductContainment - then CategoryHasSoldOutOfProducts should be one of its functions.
Since you're asking for opinions, here is mine:
(Disclaimer: That's probably something you cannot easily implement in the business world)
As you are using the term "class", I assume you want to have something object-oriented. The problem is, a service is nothing a valid object could be created from. Instead, it's just a namespace for functions.
Additionally it's very general. It's like calling a class "Manager". You can put possibly everything inside of it and this class has the potential to grow to have hundreds of functions.
My advice: Create small entities. Small enough to be created without the use of any setters, just by calling the constructor. If you notice your object needs more functionalities, create a decorator that is a little bit smarter and can do the work for you.
I would need a few more details about your environment to be more precise, but I guess in your case, you would have something like a Category class that contains products and knows when it's sold out. Just imagine you have a team of persons and everyone knows something. Ask the right guys to do the stuff and stay away from managers or services.
I've read through a bunch of tutorials to the best of my ability, but I'm still stumped on how to handle my current application. I just can't quite grasp it.
My application is simply a read-only directory that lists employees by their company, department, or sorted in alphabetical order.
I am pulling down JSON data in the form of:
Employee
Company name
Department name
First name
Last name
Job title
Phone number
Company
Company name
Department
Company name
Department name
As you can see, the information here is pretty redundant. I do not have control over the API and it will remain structured this way. I should also add that not every employee has a department, and not every company has departments.
I need to store this data, so that it persists. I have chosen Core Data to do this (which I'm assuming was the right move), but I do not know how to structure the model in this instance. I should add that I'm very new to databases.
This leads me to some questions:
Every example I've seen online uses relationships so that the information can be updated appropriately upon deletion of an object - this will not be the case here since this is read-only. Do I even need relationships for this case then? These 3 sets of objects are obviously related, so I am just assuming that I should structure it this way. If it is still advised to create relationships, then what do I gain out of creating those relationships in a read-only application? (For instance, does it make searching my data easier and cleaner? etc.)
The tutorials I've looked at don't seem to have all of this redundant data. As you can see, "company name" appears as a property in each set of objects. If it would be advised that I create relationships amongst my entities (which are Employee, Company, Department), can someone show me how this should look so that I may get an idea of what to do? (This is of course assuming that I should use relationships in my model.)
And I would imagine that this would be the set of rules:
Each company has many or no departments
Each department has 1 or many employees
Each employee has 1 company and 1 (or no) department
Please let me know if I'm on the right track here. If you need clarification, I will try my best.
Yes, use relationships. Make them bi-directional.
The redundant information in your feed doesn't matter, ignore it. If you received partial data it could be used to build the relationships, but you don't need to use it.
You say this data comes from an API, so it isn't read-only as far as the app is concerned. Worry more about how you're going to use the data in the app than how it comes from the server when designing your data model.
I have a couple of Core Data entities... Student and Exam.
Now, the Exams is initially just one object per exam - Maths Exam 3, English Exam 2 etc.
There is a relationship between Students and Exams in my data model (a student can have several exams). But initially, the Exams are just floating free, and not attached to any students.
How would I make a copy of one of the exams and attach it to a student?
If I do something like:
[student addExamsObject:examObject];
...then I think it simply references the original exam to the student, rather than making a copy.
I need a copy because the Exam has a boolean 'hasTaken', which is YES when the student has taken the exam. But if I set that now, it will make it seem like all the students with that exam have taken it.
Clarification: I would rather not restructure my model. The data is taken from a couple of xml files, one each for Students and Exams, which are parsed into the Core Data store. For instance, an Exam object might look like this:
name:Maths 5
class-id:12
year-id:4
student-id:0
..with a Student object looking like
name: Dave
class-id:12
year=id:4
student-id:222
Various rules are meant to guide which exams get attached to which students... for instance if all the Exam's ids are 0 then all students take the exam. If class-id and year-id match, and student-id is 0, then the Exam gets added to students with the same class and year. If the student-ids match, then just that student takes the Exam. etc etc.
I cannot change the way the xml is outputted from the server.
Another issue is that Exam has too-many relationship to a Question entity... in other words, the questions in the Exam. And I have to store answers to the questions that each student gives in an exam.
Edit: I wish people would try to answer my question rather than tell me to restructure my whole program. There are reasons why the data model has been structured like it is.
Edit2: Maybe I will have restructure....
Exam shouldn't have a hasTaken property. Think about it in the real world. An Exam would not know about who has taken it because many people could have taken it. The instance of taking an exam, then, should be a first-class concept in your model.
Consider this:
Exam has many TakenExams, TakenExams belongs to Student http://yuml.me/6627495d
Now the concept of taking an exam is a real object, you can then model assocation metadata as well, such as dateTaken, score, and so on.
Also remember that Core Data expects you to have all of your inverse associations set up as well.
You don't usually copy an entity. (I'm not sure what happens if you call copy on an NSManagedObject... it's not explained in the documentation, as far as I know. Experts can correct me. )
Just create another entity, or write a method which does just that.
I think another way is to make many-to-many relationships between Exam and Student:
create relationships in Exam called studentsToTakeThisExam and studentsWhoTookThisExam.
create relationships in Student called examsToTake and examsAlreadyTaken.
and set up the inverse relationships accordingly.
I would not argue (as You requested) if your modeling is correct or not. The procedure to copy an entity is, in general, quite complex, owing to the fact that, besides attributes, you also need to deal with the entity's relationships and copy them. I can not post here a huge amount of source code showing how to accomplish this, however, I can point you to a book where this issue is described in detail, with all of the source code you need. The book is the one from Marcus Zarra, "Core Data Apple’s API for Persisting Data on Mac OS X" by "The Pragmatic Programmers".
You really don't want to copy an Exam in this situation. You'd end up with lots of identically named Exams which didn't have a relationship with each other, and then you'd be forced to group them together (if you wanted to) by their name.
I'd recommend a new entity (perhaps "ExamSitting"?) which represents a Student sitting an Exam. You could then a to-many from Student to ExamSitting, and a to-many from Exam to ExamSitting. This enables you to have as many attributes on the ExamSitting as you like then (hasTaken, grade and so on).
Edit
Okay, given your clarification, I have a point or two to add (although they may not be what you're looking for). I understand that you're loading from files with a particular structure, but that doesn't necessarily have to dictate your structure.
With the XML files laid out as you now describe, I would still use an Exam - Student - ExamSitting model. If I were to implement it, I'd load all the Students, and then, for each record in the Exams file, I'd create one Exam object, and then a number of ExamSitting objects, one for each Student that fits the criteria defined in the record. As I mention above, this enables you to store more information about each event, such as mark, takenDate and so on.
If you're sure there's no requirement to be able to store additional information at this granularity, you could just create a to-many relationship studentsTakingExam. This could be populated as you load each exam record by querying the loaded Student entities.