After working with MVC for a long time, I decided to go with MVVM. I understood the basics of the pattern and got through multiple articles that explain that MVVM is waaay better then MVC any day. And I am okay with that.
I decided to make my own app in order to set my mind correctly for logic behind MVVM. So I created basic app that does follow MVVM principles and after a while I found the problem that you see in the title.
So, basically, this is the problem. Let's say that I have one object, call it Person. Person have name and surname. But when I want to show details about that person, I will have address, phone numer etc. Because one person can have many phone numbers I will have something from API that link to the user ID.
So we came to my question. If I have some basic information about some model, and want to have detail information about that same model, where do I keep ID (or link) for that detail information? Do I have to keep it inside view controller, which would be just wrong? Or do I keep it inside view model, even if I don't use it really on user interface?
The ID also belongs to the model class. ie If you have an object Person then simply create a data class Person, which will obviously include all the members say ID, Name, Address, Number and so on. You can identify each person using the same ID as well.
The View Model need not always know about the ID. If you have a list of Person objects in view model, then you could easily map each item using the ID. Additionally if you want to have currently selected item or something, you could map it to viewmodel property of that object type ie Person. So you need not keep a PersonID field in ViewModel unless it is absolutely required for some rare cases.
Sorry, but I did not understand this : So basically in prepareForSegue method I could say something like give me from current VM object at particular index and create VM for new view that I will actually send ?
As far as simple applications are concerned, the above approach is more than enough. But in some rare cases, you may need to keep the current selected item's ID in the view model. So if you're using a list and keeping a property for selected item, it may not be the type of that list ie Person. Instead it could be the ID alone.
Hope you got the point.
Related
I am struggling with something I think should be basic but cannot figure out. I have two entities in core data with a one to one and one to many relationship. They are Company which can have multiple Opportunities.I want to load a table view listing the opportunities (sorted by name) with their associated companies. Can this be done by simply accessing the Opportunity entity? If so, how do I access company? The Opportunity class references it as a "Company" type and so I tried to go using dot notation through to company.companyName but it failed on that, and if I change it to simply company (of type Company) it does show .Company: and other reference data but not the simple name field I am looking for. This seems as if it should be simple but...........
This was simple and I was overlooking the ability to load the fetchedresultscontroller with the right type (in my case the Opportunity class) and then use dot notation from there. I was trying to do it with key value access which did not work. Cheers
Is it possible to safely programmatically get a list of fields that are in the View that has just posted back to a Controller?
I noticed a problem with the default implementation of the scaffolding, in
DB.Entry(model).State = EntityState.Modified
DB.SaveChanges()
The problem is that if I haven't included a field to be edited in the view, it is being overwritten by the default value of the field that .NET assigns when creating the object. eg. If I have a User class with ID, Email and PasswordHash and I want to allow the user to update their Email address only, if I don't include anything for the PasswordHash field, it is reset to NULL as it is passed into the controller as NULL. At the moment, I am working around it by retrieving the current object from the database and updating only the fields which I know are in the View from the model passed in. That isn't such a problem for a small table, but I would like to have a general solution that I can apply across the board, especially for large tables which may during development and I don't want to have to update the code every time.
I know that I could loop through the POST variables and examine them to see what has been posted, but that creates a security issue as the user could inject additional fields that I don't want them to edit. I suppose I could explicitly exclude ones that I don't want them to edit, but then again, I would rather not have to list those if I can avoid it as it is an extra thing to maintain.
I think that there are 2 problems here and I'm not sure either are solvable...
Getting the View that posted back
Establishing which fields are included in that View (I might need to construct it again temporarily to do that?)
I suppose that I can probably get away with ignoring the first one as I could just only ever use that method on the Controller for a single View. That is still a little less neat than I'd like, but it does reduce the issue to just establishing which fields are in the View.
If a view needs only certain properties, create an interface with only those properties. Use this interface in the HttpGet and HttpPost methods.
And then you can use something like AutoMapper to map the viewmodel to your entity.
Lets say I have a view that accepts a Person object.
Has three properties, FirstName, LastName, Age
Now lets say I add another textbox field that's not part of the object.
I don't need the value of the textbox, its just populated with data that's for the user.
When you edit the fields and post the Person to the controller, lets assume there is a validation problem so you return the Person object back with Errors
The problem is now the additional textbox has lost it's value since its not part of the model.
So I made a ViewModel with a string property for that field and a Person property to keep all the values. Seems like there would be a better way to keep the value in the "special" textbox?
You should be able to get that extra field from the posted fields. How do you set it first time, through the ViewBag? You should be able to set it again.
But what exactly is wrong with using a ViewModel? Sooner or later you will have 2 or 3 extra fields, or a Person and an Appointment.
I think that's totally the right way to do it. The viewmodel is the model for the view not the model for your non-UI processing, it contains a Person and extra viewable information. It fits exactly with the concept. Your Person is presumably a (non-view) model and therefore when you have a valid post back, you get the Person to save it's data (or whatever) and the extra viewable information is irrelevant at that point, because you are no longer in a 'View/UI' part of your app.
Make the view strongly typed to your viewmodel and access the Person within it
#model myViewModel
#Model.Person.FirstName
#Model.OtherViewOnlyValue
Go with the viewmodel, so much cleaner than ViewBags/Session/ViewData etc.
There are many times that you might think that you do not need to include a UI mapping to a ViewModel but most of the time you will end up adding the mapping into the ViewModel. I believe that ViewModel should represent everything on your UI screen. Since HTTP is stateless the post form values will play an important role in populating the user interface controls.
A View accepts one Model.
But I need to draw HTML input controls for two models.
An example will illustrate:
I have a screen where I add Employees.
After adding their first name, last name and so on, I need the user to choose a number of Companies the Employees could be in.
The Companies are in one table.
The Employees are in another.
And a linking table joins them.
So it seems I need to pass the Companies to the View.
Can I pass multiple models to the view?
Or do I have to do an ugly database lookup in the View to find the Companies and manually spit out HTML for checkboxes without HTML helpers?
A Model doesn't have to consist of just one object or a single collection of one type of object. It can contain many objects and/or collections of objects. It seems that the model required for your page consists of at least collections of both employees and companies. If you have no type which fits this bill in your business object abstraction then you need to create a ViewModel for this page which can do the job.
This answer may help to explain how a ViewModel fits in MVVM ViewModel vs. MVC ViewModel
This is not entirely obvious - I'm sure it is to some guru types but for the rest of us trying to work things out its a bit more interesting.
Without going into detail I see a number of ways of solving this:
You need both sets of data so you need a view specific View Model
Your model is the Employee but you can still add other data to the ViewData - so make the Employee the model and pass the company data in as well as view data but not part of the model
(You may want to do this regardless of what model you use) Render the company selection elements as a separate view - which is where things get interesting - you obviously have to pass the existing selection, or a means to identify same, to the component view but do you have to pass the company list or could you, at this point, cheat a bit (prgamatically)? My feeling is that this is a partial view and that you need to pass it a company selection model (list of companies with selection indicated) but you could in theory do RenderAction and have an action that returns a view that goes to get the company selection for the specified employee - that way the overall view never sees the company data, that becomes a separate encapsulated problem - at least in terms of loading the data.
I think in this case where you're adding the employee either tweaking the model or adding the company list as supplementary data to the ViewData is sufficient, but for editing - assuming its required - rather than inserting you want a company selection list (all the companies with flags to indicate which are currently selected) rather than just a list of companies and at that point it all gets a bit more interesting
I want to use an example to explain what I want.
Assume I've following DB design:
Item (id, name, categoryID);
Category (id, name);
When user wants to create an Item (fill in form), I'll give a list of categories in a dropdownlist, and when user chooses one of the categories ASP.NET MVC will automatically bind categoryID, to the selected one. I need to present same dropdown list when editing the item with correct selected one.
Question:
But my DB is very big, and it requires around 30-40 (maybe even more) category-like tables, that contain just "id" and "name", and all tables need to be shown in dropdown list while creating some other object, and also needs to be presented while editing the object. Definitely above schema doesn't work, because it's tedious to write same logic 100 times with just different table names. (I'm using Linq2SQL)
Currently my solution is:
Make a view that's based in all such tables and in application I just call a function that construction dropdownlist from that single view. But it's still tedious to change view definition everytime I add a new table.
Do you guys think of a better solution for this tedious work, possibly using reflection or some other tecnologies.
It is not a problem "Definitely above schema doesn't work, because it's tedious to write same logic 100 times with just different table names."
If I were you, I will mark an addition interface on these class using "partial class" feature.
Then, I will write few extension method for the partial class.
If anyone interested in the solution:
I've used reflection to solve this problem.
I use reflection over DataContext to get the Table (by string name), and get its fields and construct the optionlist.