I have been reading a lot about the importance of keeping layers separately and the concepts are mind blowing :) . However,I looked into many places to find a practical example on how to separate these three types of models and had no luck. How do I connect them together in a practical situation? Here are few questions I have
My understanding is that persistence models contain the POCOs that we use to create entities , domain models contain the business logic and view models are used to expose relevant data. Am I correct?
If I am correct , how do I actually connect these different layers in a single solution. I mean at the end of the day , they are classes and how do I connect domain class(model) with a relevant persistence (model) and the view model?
Can someone please point me to a great tutorial that teaches how to build n-tire asp.net applications?
If not, and if possible can someone tell me how to build and connect three types of models for the following scenario.
A Customer has a Name , DOB , Address and a collection of Orders
An Order has a Customer, Date , Price and a Description
How do I create separate persistence, domain and view models(eg: A view model to display all information about customer except DOB and all orders related to the customer )
More importantly how do I connect them?
Thanks heaps!
Cheers!
you create viewmodels to match the view, not every property from the model (Entity) you might want to show in the Create/Edit view, or you want to show it but combined or in a different way, that's why you need viewmodels, usually you would have a property in the viewmodel for each editor/input in the view
for example in the Entity you can have one DateTime property but in the ViewModel 2 properties one for Date and one for Time,
or in the Entity you will have property of type Country but in the ViewModel of type int (the value of the id)
and when you have this separation you need to map Entities to ViewModels and the other way around
for demo app you can look here: http://prodinner.codeplex.com/
Related
I have been researching asp.net MVC project structure's for a new project and have a question about something is confusing me. What is the difference between models and view models? Would I be correct in saying that view models encompass models in the form properties?
I've a blog where I want to display the list of the latest posts, latest comments, post categories in a single view. How I can do that? I can strongly type my view to any one of the model right? There comes view model.
I created a view model called BlogViewModel that contains latest posts, latest comments and other stuff as properties and I bind my view with this model. The posts, comments.. are domain models while the BlogViewModel is the view model I created specially for the view.
Tomorrow I'll show my blog in a mobile version and at that time I may create a simple view model that contains only less properties. Finally.. view models are for views and most of the times they acts as wrappers over the real domain models!
A model is usually more closely related to how your data is stored (database, services, etc.) and the model will closely resemble those.
The ViewModel on the other hand is closely related to how your data is presented to the user. It is usually a flatten version of your model, denormalized, etc. It can be the aggregation of multiple models.
For your typical Person objects, your model may contain properties like the following:
FirstName
LastName
BirthDate
However, in your ViewModel you may choose to represent it differently and have something more like:
FullName
Age
ViewModel is the version of a Model from the business-domain layers adjusted to the specific View.
It has only Properties relevant to the view and shouldn't have methods(except simple ones like ToString()).
ViewModel is a "data container" only.
A model is simply a representation of an object in your application. However, there are a few different types of models that you should be aware of.
Domain Model: This represents a domain object in your application, like a SQL table if you are using an ORM (Linq2SQL, EF).
View Model: This represents an object that you want your end users to view/edit/etc. A view model could contain properties from several or no domain models and typically exclude properties that your end users should not be mucking with. View Models should only contain the elements that are needed to display the appropriate data to the end user for a specific request.
Here is some Jimmy Bogard for you about View Models and their use.
I have read online that it is bad practice to use a "kitchen sink" model:
Rule #3 – The View dictates the design of the ViewModel. Only what is
required to render a View is passed in with the ViewModel.
If a Customer object has fifty properties, but one component only
shows their name, then we create a custom ViewModel type with only
those two properties.
Jimmy Bogard's subsequent explanation of how this is good, however, left me a little questioning. It'd be so easy to have my Model just contain a list of Customers, I could even use my POCO's.
So now I get to create custom little view model fragments for every page on the site? Every page that uses a Customer property would get one, but of course could not be shared since some of the information is extraneous, if one page used Age but not Name, for example. Two new mini view model classes right?
This is very time consuming, and seems like it'll lead to a million little custom view models - can someone elaborate as to the utility of this approach and why the easier approach is bad?
View model class can be used not only to transfer values, but it also defines data types (data annotations), validation rules and relations different then ones used in model. Some advantages that come to my mind right now:
There are different validation rules when you change user's password,
change his basic data or his subscription setting. It can be
complicated to define all these rules in one model class. It looks
much better and cleaner when different view models are used.
Using view model can also give you performance advantages. If you
want to display user list, you can define view model with id and name
only and use index to retrieve it from database. If you retrieved
whole objects and pass it to view, you transfer more data from
database than you need to.
You can define display, and editor templates for view models and reuse them on different pages using html helpers. It looks much worse, when you define templates for model POCOs.
If you would use your POCO objects as view models, you would essentially be showing your private objects and break the encapsulation. This in turn would make your model hard to change without altering the corresponding views.
Your data objects may contain details that are appropriate only to the data access layer. If you expose those things to the view, someone might alter those values that you did not expect to be altered and cause bugs.
Many of the same reasons as for having private members in OO languages apply to this reasoning. That being said, it's still very often broken because it's a lot of extra work to create all these "throw-away" models that only gets used once. There exists frameworks for creating these sorts of models, though the name eludes me, that can tie objects together and pick out the interesting properties only which takes away some of the drudgery from creating specific view models.
Your View Model tells the View how data should be shown. It expresses the model. I don't think its necessary to have two view models unless you have two ways to express your model. Just because you have two pages, doesn't mean you will be showing the data any different way, so I wouldn't waste time making two mini View Models when it can be in one reusable view model, Imagine if later you have a page that needs Name and Age, you would create another view model? It's absolutely silly. However, if you had two pages both showing 'Age' and it needed to be shown in a different way, then I would create another one.
I'm coming to the end of my first MVC project, and I'm not overly happy with how I constructed my Model objects and I'm looking for some ideas on how to improve them.
I use repositories for each DB table with Get, Save, Delete etc methods.
The repositories use Linq2Sql for the DB access.
I do mapping from the Linq2Sql objects to MVC Model objects, in the main, these are very much 1 to 1 mappings.
My problem is, I don't think my MVC model objects were granular enough, and I am probably passing more data back and forth than needed.
For example, I have a User table. An admin can edit a users details as can the user themselves, so I reckon I should really have a "AdminUserModel" and "UserModel" objects, where "AdminUserModel" has a greater set of values (IsEnabled for example).
So my bigger question is really, what kind of architectures are people using out there in the wild, in order to map many similar, related Model objects down through the layers to the DB?
Any sample architecture solutions anyone can suggest beyond NerdDinner?
thanks in advance!
In the case of your user model, you should use inheritence in stead of 2 seperated models. In this way you can use the code that was created for user in the ones that inherite from it.
the type of model you use depends completely on what you want to do with it. A good thing might be to take a look at patterns and try to get the patterns working that are needed for your situation...
I usually take implement inheritance in my models.
I usually have a base class of entity, which will have id, datecreated, valid and any other fields that are shared between entities (publishStatus, locked etc).
If needs be you can create other base classes inheriting from entity: person entity, product entity etc.
this way you can have a generic repository base, constrained to Entity or IEntity, i find that most entities CRUD functions dont need much more behaviour than that provided by the generic base (perhaps you will need to add a few additional get methods for some types)
In your case, AdminUser could inherit from User
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
How do most developers handle Typed Views in ASP.NET MVC when dealing with large applications? We are considering putting View-specific models in the Models folder, then putting all domain objects into a separate project. This way our Controllers can easily add the domain object to the typed view, without the domain object needing to be aware of the View layout itself.
For example, if we have an Employee object with:
Id
First Name
Last Name
Status
Then our Employee View might use a ViewEmployeeModel object with:
Employee object
List to populate Status drop-down
etc
Is this a sensible approach? Are there better ways to accomplish the same thing? It seems a little strange since I'd basically have two models (one for the view, one for the business objects), but isn't it better than using untyped views?
I do this almost as a rule, because:
It lets you design the app view-first instead of DB-first, which is nice when working with customer representatives.
Views typically have a much "flatter" object graph than, say, Entity Framework models. LINQ makes it easy to map these.
The views and the data model can evolve much more independently.
It's typically easier to model bind to a flat view model with, say, FK IDs than an entity model which expects fully materialized related objects.
You don't have to worry about accidentally exposing "secret" properties or whitelisting properties for updates.
Don't have the reputation to comment, but Craig is right. It's a variation of the Model-View-ViewModel pattern. There's a good article on it available at Los Techies.
The article also uses the AutoMapper code that mgroves pointed out so you should be able to kill two birds with one stone.
I think this is a pretty sensible approach. One thing that might help you out is AutoMapper.
Seems fine, you have the advantage that the model only contains information needed by the view and not lots of pesky business logic functions / values.