This is sort of a generic question in regards to mapping between data entities, domain objects, and ViewModels. I may not be asking it right but hopefully I can make some sense of it. Below is a simplified problem.
Pretend I have an Entity Framework model which maps 1:1 to my database tables, but my domain objects may not be identical, and my ViewModel is drastically different again. As a pseudo-example:
Database/EF Entities:
MembershipAccount
MembershipAccountExtraInfo
Domain:
Account
Profile
Preferences
ViewModel:
UserProfileModel
Let's say I need to display a UserProfileModel which has: Username (from MembershipAccount), SignupDate (from MembershipAccount), FullName (from MembershipAccountExtraInfo), and TimeZone (from MembershipAccountExtraInfo)
What sort of relationships might I need here, and what sort of mapping mechanisms? Is it common to have something like an AccountMapper that takes both a MembershipAccount and MembershipAccountExtraInfo and returns an Account? I'm a bit stuck on the mapping when several objects are needed to create a single domain entity, and vice versa.
If it helps: I'm designing an API for managing User Accounts, User Profiles, User Preferences, etc. but the database tables are all over the place. A single User Profile might need to be created from data spanning 4-5 tables and 2 databases. There is no 1:1 mapping between my database tables and any (logical) domain objects.
Thanks!
I like to work keeping my domain objects as close to the objects that they represent as possible. What I mean by this is that if an account has preferences, then the domain Account object should contain a Preferences property, most likely represented by a collection of Preference objects. If nothing else, this helps the users understand the data structure of the application easily.
As for constructing the view models, that's the easiest bit... you add just properties for anything that is required. What types of properties you would need would really depend on how you have structured your domain objects.
If your view has the requirements that you mentioned in your question and you modelled your domain objects closely on the objects that they represent, then by the sounds of it, you would just need an Account object because that would contain the Preference and Profile objects inside it.
Finally, the only 'mapping' that needs to be done can be done with a LinQ query using the Entity Framework. It is at this point that I join the tables and pull whatever data that I need for whichever object I am working on. Here is an example of instantiating objects from data from three tables (using LinQ2SQL):
public AudioTracks GetAudioTracks(AudioTrackSearchOptions searchOptions)
{
AudioTracks audioTracks;
using (MidasDataContext dataContext = DataContext)
{
audioTracks = new AudioTracks(
from audioTrack in dataContext.DbAudioTracks
join masterTrack in dataContext.DbMasterTracks on audioTrack.MasterTrackId equals masterTrack.Id
join masterTrackArtist in dataContext.DbDataLists on masterTrack.ArtistId equals masterTrackArtist.Id
orderby string.Concat(masterTrack.Title, " (", audioTrack.Mix, ") - ", masterTrackArtist.Text)
where (searchOptions.IsInactiveAudioTrackIncluded || audioTrack.IsActive)
&& (searchOptions.IsDeletedAudioTrackIncluded || !audioTrack.IsDeleted)
select new AudioTrack(audioTrack.Id, masterTrack.Id, audioTrack.Isrc, masterTrack.Title, masterTrackArtist.Text, audioTrack.Mix, audioTrack.IsContentExplicit, audioTrack.IsActive, audioTrack.IsDeleted));
}
audioTracks.Sort(a => a.TitleWithMix);
return audioTracks ?? new AudioTracks();
}
UPDATE >>>
Extending my AudioTracks example and working backwards, the GetAudioTracks method is in a project called DataProviders. It is called from a GetAudioTracks method in a DataController class which just adds user feedback and re-try options. That in turn is called by a TracksModel in the Models project which just contains a subsection of methods from the DataController class that relate to the various types of tracks in the application.
Finally, the AudioTracksViewModel in the ViewModels project calls the TracksModel.GetAudioTracks method upon initialisation which happens when the AudioTracksView is loaded by the user. The AudioTracksView has a ListBox on the left containing all of the AudioTrack objects that meet the users search and/or filter selections. The right of the screen has the fields for the selected AudioTrack. Here is what it looks like (if the link seems broken, you can view the image here):
The more transparent fields with an edit Button on the right are read only fields connected to collections. The edit Button opens a dialog to let the user enter multiple items, which are then summarised in the field. All of the objects in the application have similar views of more or less complexity.
Related
Vaughn Vernon has a series of pdfs that suggests to use indirect references to link related entities together. This way, Entity A would have a list of Entity B Ids instead of references to them. This avoids performance and scalability issues and also let's you focus on consistency boundaries.
My problem comes with traversing such a type of indirect reference to be able to perform domain logic.
Say I have Group, User and Authorizations. All three are entities and also their own aggregate. They can each be modified simultaneously from one another, and therefore are not part of a consistency boundary. They relate to each other in the way that Users are in Groups and Groups are Authorized for things.
I need domain logic that retrieves me the list of things a User is Authorized to. Ideally, I feel like this logic should be on the User as User.GetAuthorizations(); But I'm willing to move this somewhere else if it feels more appropriate.
The problem is that, no matter where I put this logic I will need to:
Get the Groups of a User.
For each of the user's groups, get
the given Authorizations.
Perform the merge logic of those
Authorization.
Return the Authorizations as they fit the User.
How should I therefore implement User.GetAuthorizations(); in a DDD context which uses Repositories for data access?
By this I mean that I want to know how I should retrieve the indirect reference and traverse it inside my method.
Here is an example:
public class UserApplicationService
{
public IEnumerable<Authorization> GetUserAuthorizations(string userId)
{
User user = _userRepo.Find(userId);
IEnumerable<Group> groups = _groupRepo.FindMany(user.GroupIds);
List<Tuple<Group, List<Authorization>>> groupAuths = new List<Tuple<Group, List<Authorization>>>()
foreach(Group group in groups)
{
List<Authorization> auths = _authorizationRepo.FindMany(group.AuthorizationIds).ToList();
Tuple<Group, List<Authorization>> groupAuth = new Tuple<Group, List<Authorization>>(group, auths);
groupAuths.Add(groupAuth);
}
return user.GetAuthorizations(groupAuths);
}
}
IEnumerable<Authorization> User.GetAuthorizations(List<Tuple<Group, List<Authorization>>> groupAuths)
{
// merge logic would be here
}
In this case, I retrieve in an Application Service the groups of the User and the Authorizations of all of those groups and I pass it to the User.GetAuthorizations method. I find this quite cumbersome, and if the relationship would be nested deeper, it would become even more cumbersome.
I wonder what other approach would there be that respect the DDD approach? How is this normally done?
Time has passed since asking this, and I've furthered my understanding to the point where I feel confident in answering this myself.
I need domain logic that retrieves me the list of things a User is Authorized to. Ideally, I feel like this logic should be on the User as User.GetAuthorizations(); But I'm willing to move this somewhere else if it feels more appropriate.
This is where lies the entire problem of my question, I want retrieval logic, and I'm mistakenly thinking it must go in my domain.
In DDD, you have a domain model that tries to encapsulate the ubiquitous language of the application, but only for purposes of data change. Those entities, value objects and aggregate roots should encapsulate transactional context.
When you want to get information, you bypass all that. Query your data directly, and get exactly the info you want in the form you need.
The problem is that, no matter where I put this logic I will need to:
Get the Groups of a User.
Perform the merge logic of those Authorization.
Return the Authorizations as they fit the User.
How should I therefore implement User.GetAuthorizations(); in a DDD context which uses Repositories for data access?For each of the user's groups, get the given Authorizations.
This was missing the point. The proper way to do this is have an application service that has a method GetAuthorizations, like in my example, but this method should never use the Repository and never use the Aggregate Roots User, Group and Authorization. Instead, it should use a second abstraction, which is a query layer for retrieval of persistence data, which returns it's own type of entities that better match the query. So GetAuthorizations could retrieve objects of type AuthorizedUser. It would implement the merge logic as part of the querying process.
First of all - make sure that customer actually needs grouping of users for authorization rules definition. If not, your approach is an overkill and consider defining authorization rules in static fashion.
Another approach would be to make UserAuthorization entity that gets properly mutated for changes like 'user group has changed', 'authorization deleted' etc. Then it would be simple as asking authorization rules list for a user.
I currently have a repository for just about every table in the database and would like to further align myself with DDD by reducing them to aggregate roots only.
Let’s assume that I have the following tables, User and Phone. Each user might have one or more phones. Without the notion of aggregate root I might do something like this:
//assuming I have the userId in session for example and I want to update a phone number
List<Phone> phones = PhoneRepository.GetPhoneNumberByUserId(userId);
phones[0].Number = “911”;
PhoneRepository.Update(phones[0]);
The concept of aggregate roots is easier to understand on paper than in practice. I will never have phone numbers that do not belong to a User, so would it make sense to do away with the PhoneRepository and incorporate phone related methods into the UserRepository? Assuming the answer is yes, I’m going to rewrite the prior code sample.
Am I allowed to have a method on the UserRepository that returns phone numbers? Or should it always return a reference to a User, and then traverse the relationship through the User to get to the phone numbers:
List<Phone> phones = UserRepository.GetPhoneNumbers(userId);
// Or
User user = UserRepository.GetUserWithPhoneNumbers(userId); //this method will join to Phone
Regardless of which way I acquire the phones, assuming I modified one of them, how do I go about updating them? My limited understanding is that objects under the root should be updated through the root, which would steer me towards choice #1 below. Although this will work perfectly well with Entity Framework, this seems extremely un-descriptive, because reading the code I have no idea what I’m actually updating, even though Entity Framework is keeping tab on changed objects within the graph.
UserRepository.Update(user);
// Or
UserRepository.UpdatePhone(phone);
Lastly, assuming I have several lookup tables that are not really tied to anything, such as CountryCodes, ColorsCodes, SomethingElseCodes. I might use them to populate drop downs or for whatever other reason. Are these standalone repositories? Can they be combined into some sort of logical grouping/repository such as CodesRepository? Or is that against best practices.
You are allowed to have any method you want in your repository :) In both of the cases you mention, it makes sense to return the user with phone list populated. Normally user object would not be fully populated with all the sub information (say all addresses, phone numbers) and we may have different methods for getting the user object populated with different kind of information. This is referred to as lazy loading.
User GetUserDetailsWithPhones()
{
// Populate User along with Phones
}
For updating, in this case, the user is being updated, not the phone number itself. Storage model may store the phones in different table and that way you may think that just the phones are being updated but that is not the case if you think from DDD perspective. As far as readability is concerned, while the line
UserRepository.Update(user)
alone doesn't convey what is being updated, the code above it would make it clear what is being updated. Also it would most likely be part of a front end method call that may signifiy what is being updated.
For the lookup tables, and actually even otherwise, it is useful to have GenericRepository and use that. The custom repository can inherit from the GenericRepository.
public class UserRepository : GenericRepository<User>
{
IEnumerable<User> GetUserByCustomCriteria()
{
}
User GetUserDetailsWithPhones()
{
// Populate User along with Phones
}
User GetUserDetailsWithAllSubInfo()
{
// Populate User along with all sub information e.g. phones, addresses etc.
}
}
Search for Generic Repository Entity Framework and you would fine many nice implementation. Use one of those or write your own.
Your example on the Aggregate Root repository is perfectly fine i.e any entity that cannot reasonably exist without dependency on another shouldn't have its own repository (in your case Phone). Without this consideration you can quickly find yourself with an explosion of Repositories in a 1-1 mapping to db tables.
You should look at using the Unit of Work pattern for data changes rather than the repositories themselves as I think they're causing you some confusion around intent when it comes to persisting changes back to the db. In an EF solution the Unit of Work is essentially an interface wrapper around your EF Context.
With regards to your repository for lookup data we simply create a ReferenceDataRepository that becomes responsible for data that doesn't specifically belong to a domain entity (Countries, Colours etc).
If phone makes no sense w/o user, it's an entity (if You care about it's identity) or value object and should always be modified through user and retrieved/updated together.
Think about aggregate roots as context definers - they draw local contexts but are in global context (Your application) themselves.
If You follow domain driven design, repositories are supposed to be 1:1 per aggregate roots.
No excuses.
I bet these are problems You are facing:
technical difficulties - object relation impedance mismatch. You are struggling with persisting whole object graphs with ease and entity framework kind a fails to help.
domain model is data centric (as opposed to behavior centric). because of that - You lose knowledge about object hierarchy (previously mentioned contexts) and magically everything becomes an aggregate root.
I'm not sure how to fix first problem, but I've noticed that fixing second one fixes first good enough. To understand what I mean with behavior centric, give this paper a try.
P.s. Reducing repository to aggregate root makes no sense.
P.p.s. Avoid "CodeRepositories". That leads to data centric -> procedural code.
P.p.p.s Avoid unit of work pattern. Aggregate roots should define transaction boundaries.
This is an old question, but thought worth posting a simple solution.
EF Context is already giving you both Unit of Work (tracks changes) and Repositories (in-memory reference to stuff from DB). Further abstraction is not mandatory.
Remove the DBSet from your context class, as Phone is not an aggregate root.
Use the 'Phones' navigation property on User instead.
static void updateNumber(int userId, string oldNumber, string newNumber)
static void updateNumber(int userId, string oldNumber, string newNumber)
{
using (MyContext uow = new MyContext()) // Unit of Work
{
DbSet<User> repo = uow.Users; // Repository
User user = repo.Find(userId);
Phone oldPhone = user.Phones.Where(x => x.Number.Trim() == oldNumber).SingleOrDefault();
oldPhone.Number = newNumber;
uow.SaveChanges();
}
}
If a Phone entity only makes sense together with an aggregate root User, then I would also think it makes sense that the operation for adding a new Phone record is the responsibility of the User domain object throught a specific method (DDD behavior) and that could make perfectly sense for several reasons, the immidiate reason is we should check the User object exists since the Phone entity depends on it existence and perhaps keep a transaction lock on it while doing more validation checks to ensure no other process have deleted the root aggregate before we are done validating the operation. In other cases with other kinds of root aggregates you might want to aggregate or calculate some value and persist it on column properties of the root aggregate for more efficient processing by other operations later on. Note though I suggest the User domain object have a method that adds the Phone it doesn't mean it should know about the existence of the database or EF, one of the great feature of EM and Hibernate is that they can track changes made to entity classes transparently and that also means adding of new related entities by their navigation collection properties.
Also if you want to use methods that retrieve all phones regardless of the users owning them you could still though it through the User repository you only need one method returns all users as IQueryable then you can map them to get all user phones and do a refined query with that. So you don't even need a PhoneRepository in this case. Beside I would rather use a class with extensions method for IQueryable that I can use anywhere not just from a Repository class if I wanted to abstract queries behind methods.
Just one caveat for being able to delete Phone entities by only using the domain object and not a Phone repository you need to make sure the UserId is part of the Phone primary key or in other words the primary key of a Phone record is a composite key made up of UserId and some other property (I suggest an auto generated identity) in the Phone entity. This makes sense intuively as the Phone record is "owned" by the User record and it's removal from the User navigation collection would equal its complete removal from the database.
Here's the story: a Site (physical location of interest) has zero or more Contacts. Those Contacts are people associated with a Company who are authorized to deal with matters regarding the Site.
The schema looks like:
Person -< CompanyContact -< CompanySiteContact >- Site
||
| -< PersonPhone
|
-< PersonAddress
My entry point is Site. I need the list of Contacts. There is very little field data of interest until you get to Person. So, I'd like to collapse Person, CompanyContact and CompanySiteContact into one domain class.
The options I've come up with:
Create one domain class and use joins in the FluentNH map to flatten the layers as it retrieves the data
. It never sounded simple, and I'm running into problems with the multi-level join (if A joins B joins C, you can't specify the join to C within the join to B). I think, however, that if it's possible to specify the joins, that's just a one-time thing and so this will end up being the most maintainable solution.
Replicate the deep model in a set of "DTOs" that map 1:1 to the tables and can be passed to the constructor of a "flat" domain model. It works, but it feels like cheating (there is no problem that cannot be solved with another layer of abstraction, EXCEPT for having too many layers of abstraction), and my instinct tells me this will somehow eventually cause more problems than it solves.
Replicate the domain model 1:1 with the schema and use pass-through properties on CompanySiteContact to access properties down in the depths of a Person record. Again, works now, but it doesn't really solve the problem, and every new property that becomes of interest will require changes to the mapping, the actual domain class, AND the top-level domain class. Not very SOLID.
So, the Q is, how would I structure the mapping? Like I said, I'm not able to specify a join in a join. I think the way I have to do it is map the PK of each table, and use it in the next join from the top level, but I'm not exactly sure how to set that up (haven't used FluentNH to set up anything close to this complex before).
I'd recommend creating your domain model to closely match your database. From there I'd create DTOs and use AutoMapper to do the flattening. Easy.
Thanks to James for his answer; +1, but I don't think AutoMapper is necessary at this juncture, and I'm a little uneasy at including something that does the job quite that "automagically". I thought of a few more options:
Set up a view in the DB. This will work because due to business rules, the contact information is read-only; the app I'm developing must never update a contact directly because a different department maintains this rolodex.
Map my domain 1:1 as James suggested, but then use a Linq query to flatten the model into a DTO. This query can be encapsulated within a helper of the Repository, allowing developers to query the DTO directly using the same methods on the Repository as for other classes. It's more complex than the view with the same result, but it doesn't require schema changes.
I'll probably go with the first option, and resort to the second if necessary.
I am using NHibernate for persistence and have my domain model mapped out. I am using MVC2 and I am keen to use the built in model validation for all basic validation (string length, range, etc).
The problem is that I'm finding that there is a huge amount of work involved in this. I have 26 POCO classes representing the domain. Which means I need roughly 5 x 26 = 130 ViewModels (one for create, edit, view, list, delete) given I'm going to use ViewModels everywhere. 130 ViewModels to create and maintain considering I have my domain objects already mapped out seems wasteful.
Now I've bought into the idea that this is a good way to develop as a way to keep the view as dumb as possible, eliminate view related security blunders, etc. To be honest I get the strong impression I'm missing a tool or technique that would speed this process up.
Is there a way to reduce this pain?
I'm not sure why you would need 5 ViewModels? The ViewModels kinda represent what you want to display (in the view):
Lets say you have these Related Entities:
BlogArticle
Tags
CategoryName
You have three entities but from the form that creates the BlogArticle you're writing the article, selecting tags and then choosing a category on one page.
On the BlogArticle create form you just want the following ViewModel:
BlogArticleViewModel
+string BlogTitle
+string BlogContent
+string TagName //could be mapped to textbox, checkbox
+string CategoryName//same as above
You could use http://automapper.codeplex.com to map Models to ViewModels.
I'm reading about the Entity Framework 4.0 and I was wondering why should I create a complex type and not a new Entity (Table) and a relation between them?
The perfect example is an address. Using a complex type for an address is much easier to deal with than a new entity. With complex types you do not have to deal with the Primary Key. Think about accessing an address how many common types of entities would have an address (Business Units, People, Places). Imagine populating many peoples addresses and needing to set a key for each one. With complex types you simply access the internal properties of they type and you're done. Here is an MSDN link of an example. http://msdn.microsoft.com/en-us/library/bb738613.aspx
This question has been here a while already, but I'm going to add an answer anyway in the hopes that the next poor sob that comes along knows what he's in for.
Complex types do not support lazy loading, at least not in EF 4.3. Let's take the address situation as an example. You have a Person table with 15 columns, 5 of which contain address information for certain individuals. It has 50k records. You create entity Person for the table with a complex type Address.
If you need a list of names of all individuals in your database you would do
var records = context.Persons;
which also includes addresses, pumping 5*50k values into your list for no reason and with noticeable delay. You could opt to only load the values you need in an anonymous type with
var records = from p in context.Persons
select new {
LastName = p.LastName,
FirstName = p.FirstName,
}
which works well for this case, but if you needed a more comprehensive list with, say, 8 non-address columns you would either need to add each one in the anonymous type or just go with the first case and go back to loading useless address data.
Here's the thing about anonymous types: While they are very useful within a single method, they force you to use dynamic variables elsewhere in your class or class children, which negate some of Visual Studio's refactoring facilities and leave you open to run-time errors. Ideally you want to circulate entities among your methods, so those entities should carry as little baggage as possible. This is why lazy loading is so important.
When it comes to the above example, the address information should really be in a table of its own with a full blown entity covering it. As a side benefit, if your client asks for a second address for a person, you can add it to your model by simply adding an extra Address reference in Person.
If unlike the above example you actually need the address data in almost every query you make and really want to have those fields in the Person table, then simply add them to the Person entity. You won't have the neat Address prefix any more, but it's not exactly something to lose sleep over.
But wait, there's more!
Complex types are a special case, a bump on the smooth landscape of plain EF entities. The ones in your project may not be eligible to inherit from your entity base class, making it impossible to put them through methods dealing with your entities in general.
Assume that you have an entity base class named EntityModel which defines a property ID. This is the key for all your entity objects, so you can now create
class EntityModelComparer<T> : IEqualityComparer<T> where T : EntityModel
which you then can use with Distinct() to filter duplicates from any IQueryable of type T where T is an entity class. A complex type can't inherit from EntityModel because it doesn't have an ID property, but that's fine because you wouldn't be using distinct on it anyway.
Further down the line you come across a situation where you need some way to go through any entity and perform an operation. Maybe you want to dynamically list the properties of an entity on the UI and let the user perform queries on them. So you build a class that you can instantiate for a particular type and have it take care of the whole thing:
public class GenericModelFilter<T> : where T : EntityModel
Oh wait, your complex type is not of type EntityModel. Now you have to complicate your entity inheritance tree to accommodate complex types or get rid of the EntityModel contract and reduce visibility.
Moving along, you add a method to your class that based on user selections can create an expression that you can use with linq to filter any entity class
Expression<Func<T, bool>> GetPredicate() { ... }
so now you can do something like this:
personFilter = new GenericModelFilter<Person>();
companyFilter = new GenericModelFilter<Company>();
addressFilter = new GenericModelFilter<Address>(); //Complex type for Person
...
var query = from p in context.Persons.Where(personFilter.GetPredicate())
join c in context.Companies.Where(companyFilter.GetPredicate()) on p.CompanyID = c.ID
select p;
This works the same for all entity objects... except Address with its special needs. You can't do a join for it like you did with Company. You can navigate to it from Person, but how do you apply that Expression on it and still end up with Person at the end? Now you have to take moment and figure out this special case for a simple system that works easily everywhere else.
This pattern repeats itself throughout the lifetime of a project. Do I speak from experience? I wish I didn't. Complex types keep stopping your progress, like a misbehaved student at the back of the class, without adding anything of essence. Do yourself a favor and opt for actual entity objects instead.
Based on Domain Driven Design Concepts, Aggregate root could have one or more internal objects as its parts. In this case, Internal objects - inside the boundary of Aggregate Root - does not have any KEY. The parent key will be applied to them or somehow like this. Your answer returns to the benefit of keeping all Parts inside Aggregate root that makes your model more robust and much simpler.