I have a question about Entity Framework.
In our project we would need to have some circular references, like this one:
public class OptionClusterSet
{
public int ID { get; set; }
public virtual ICollection<OptionCluster> OptionClusters { get; set; }
}
public class OptionCluster
{
public int ID { get; set; }
public long OptionClusterSetId { get; set; }
public virtual OptionClusterSet OptionClusterSet { get; set; }
}
The thing is that whenever we try to, for example, get a OptionClusterSet including its OptionClusters using eager loading, the OptionClusters try to load their OptionClusterSets and so on. So we get an infinite loop.
Is there a way to configure this so it works properly?
It works properly out of the box unless you try to serialize it - serialization needs some special handling (attributes) to let serializer recognize circular reference.
Eager loading loads only the level you specify in Include call, nothing more. Everything else can be loaded through lazy loading but EF don't load again the relation which was already loaded. There are some scenarios when it doesn't work as expected - the example is navigation property fixup in POCO generator (it lazy loads additional data to fixup the reverse navigation property).
Related
Let's say I have 2 entity models -
public class Blog
{
[Key]
public int BlogId { get; set; }
public string BlogName { get; set; }
public virtual ICollection<Post> Posts { get; set; }
}
public class Post
{
[Key]
public int PostId { get; set; }
public string PostName { get; set; }
public int BlogId { get; set; }
[ForeignKey("BlogId")]
public virtual Blog Blog { get; set; }
}
Let's get a blog-
var blog = dbContext.Blogs.Where(r => r.BlogId == 1).FirstOrDefault();
The code retrieves the blog and all the posts as well. But if there is thousands of posts under this blog? It would be a performance issue.
Is there any trick not to load posts by default? I don't want to remove Posts property from Blog.
Thanks.
You can achieve this by two ways.
Lazy Loading
Explicit Loading
Lazy Loading - DbContext has a configuration setting that enables lazy loading DbContext.Configuration.LazyLoadingEnabled.
This setting is true by default and therefore if you have not changed the default, the dynamic proxy will perform lazy loading. For using lazy loading it need to have two things implemented in your POCO classes.
Your POCO classes must be public and not sealed.
The navigation properties that you want to be lazy loaded must also be marked as virtual (which you have already done) so that Entity Framework can override the properties to include the lazy loading logic.
2- Explicit Loading - Explicit loading is like lazy loading in that related data is loaded separately, after the main data has been loaded. However, unlike lazy loading, it doesn’t automatically happen for you; you need to call a method to load the data.
Explicit loading is achieved using the DbContext.Entry method. The Entry method gives you access to all the information that the DbContext has about an entity. This goes beyond the values that are stored in the properties of the actual entity and includes things such as the state of the entity and the original values for each property when it was retrieved from the database.
Ex:
var blog = dbContext.Blogs.Where(r => r.BlogId == 1).FirstOrDefault();
context.Entry(blog)
.Collection(d => d.Posts)
.Load();
You can control which children load with a projection:
var blogWithSomeChildren = dbContext.Blogs
.Select(b => new
{
blog = b,
selectedPosts = b.Posts.Where(p => p.PostId > 500) // or whatever criteria you want...
})
.Single(b => b.BlogId ==1);
I have found a solution that works (using DTOs and AutoMapper), which is reproduced below, but I would prefer an answer that lists the different approaches to the problem with examples and this will be marked as the answer if received.
In my entity model I have a navigation property that goes from a child entity to the parent entity. My project was working swimmingly. Then I began to use AutoFixture for unit testing, and testing failed, AutoFixture saying I had a circular reference.
Now, I realise that circular reference navigation properties like this are OK within Entity Framework, but I found this post (Use value of a parent property when creating a complex child in AutoFixture), where Mark Seemann, the creator of AutoFixture states:
"For the record, I haven't written an API with a circular reference for years, so it's quite possible to avoid those Parent/Child relations."
So, I want to understand HOW a domain model can be refactored to avoid child/parent relations.
Below are the entity classes in question, the repository method, and how I use the property causing the circular reference in my View. The perfect answer would explain the different options I could choose from with examples, and the basic pros/cons of each approach.
Note: The property causing the circular reference is User, in the UserTeam model.
Models:
public class UserProfile
{
public UserProfile()
{
UserTeams = new HashSet<UserTeam>();
Games = new HashSet<Game>();
}
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int UserId { get; set; }
public string UserName { get; set; }
public virtual ICollection<UserTeam> UserTeams { get; set; }
public virtual ICollection<Game> Games { get; set; }
}
public class Game
{
public Game()
{
UserTeams = new HashSet<UserTeam>();
}
public int Id { get; set; }
public int CreatorId { get; set; }
public virtual ICollection<UserTeam> UserTeams { get; set; }
}
public class UserTeam
{
public UserTeam()
{
UserTeam_Players = new HashSet<UserTeam_Player>();
}
public int Id { get; set; }
public int UserId { get; set; }
public int GameId { get; set; }
public virtual UserProfile User { get; set; }
public virtual ICollection<UserTeam_Player> UserTeam_Players { get; set; }
}
Repository Method
public IEnumerable<Game> GetAllGames()
{
using (DataContext)
{
var _games = DataContext.Games
.Include(x => x.UserTeams)
.Include(x => x.UserTeams.Select(y => y.User))
.ToList();
if (_games == null)
{
// log error
return null;
}
return _games;
}
}
View
#model IEnumerable<Game>
#foreach (var item in Model){
foreach (var userteam in item.UserTeams){
<p>#userteam.User.UserName</p>
}
}
Now, if I remove the 'User' navigation property, I wouldn't be able to do '#userteam.User.UserName'
So, how do I refactor the domain model to remove the circular reference, whilst being able to easily loop through Games, and do something like
UserTeam.User.Username?
I had a similar problem with AutoFixture and EntityFramework a while ago. My solution was to add an extension to AutoFixture, that allows you to build a SUT with a few recursions. That extension has recently been adopted in AutoFixture.
But I understand that your question was not about how to make AutoFixture construct recursive data structures, which is indeed possible, but how to create domain models without recursion.
First, you have tree or graph structures. Here anything but recursion would mean indirection through loose coupled node ids. Instead of defining an association, you would have to traverse the tree query-by-query or cache the whole thing and traverse by node-key lookup, which may be impractical depending on the tree-size. Here it is very convenient to make EF do the work for you.
The other common structure is a two-way navigational structure similar to your user / game scenario. Here it is often not that inconvenient to prune the navigation flow to a single direction. If you omit one direction, say from game to team, you can still easily query all teams for a given game. So: User has a list of games and a list of teams. Team has a list of games. Games have no navigational reference to either. To get all users for a specific game you could write something like:
var users = (from user in DataContext.Users
from game in user.Games
where game.Name == 'Chess'
select user).Distinct()
I have found a solution that works (using DTOs and AutoMapper), which is reproduced below, but I would still prefer an answer that lists the different approaches to the problem with examples, in particular whether this is a desirable solution, or whether I should stick with the navigation properties as they were, get rid of AutoFixture, and when it comes to serializing for json just utilise other work arounds (attributes etc)...
So, in my View Model, I added a couple of classes:
public class GameDTO
{
public int Id { get; set; }
public int CreatorId { get; set; }
public ICollection<UserTeamDTO> UserTeamsDTO { get; set; }
}
public class UserTeamDTO : UserTeam
{
public UserProfile User { get; set; }
}
And in my controller, I use AutoMapper to map the Game / UserTeam objects from the repository to my DTO objects, and return the IList _gamesDto to the View.
var _games = _gameRepository.GetAllGames();
IList<GameDTO> _gamesDto = new List<GameDTO>();
IList<UserTeamDTO> _userteamsDto = new List<UserTeamDTO>();
GameDTO _gameDto = new GameDTO();
UserTeamDTO _userteamDto = new UserTeamDTO();
Mapper.CreateMap<Game, GameDTO>();
Mapper.CreateMap<UserTeam, UserTeamDTO>();
foreach (Game _game in _games)
{
foreach (UserTeam _userteam in _game.UserTeams)
{
_userteamDto = Mapper.Map<UserTeamDTO>(_userteam);
_userteamDto.User = _userRepository.GetUser(_userteam.UserId);
_userteamsDto.Add(_userteamDto);
}
_gameDto = Mapper.Map<GameDTO>(_game);
_gameDto.UserTeamsDTO = _userteamsDto;
_gamesDto.Add(_gameDto);
}
I had a similar problem recently which also impacted serializing JSON objects. I decided to remove the circular references from my data model.
I first removed the redundant navigation properties which were creating the circular references. I made sure that my resulting tree of data made sense. This allowed me to make it clear which objects own which relationships.
This also made EF unable to automatically reason about my relationships. I had to specify the One-to-Many and Many-to-Many relationships using the FluentAPI. I found a solution here: https://stackoverflow.com/a/16719203/1887885
Hope this is helpful.
I'm using Entity Framework Code First. The class i'm trying to create contains two collections (of the same type). I'm having problem recovering my respective collections.
My classes look like this:
public class Destination
{
public int DestinationId { get; set; }
public string Name { get; set; }
public List<Lodging> Lodgings { get; set; }
public List<Lodging> Lodgings2 { get; set; }
}
public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public Destination Destination { get; set; }
}
I created a new Destination, then I reopened (closed & opened) the database connection. When I retrieve the destination, my collections (dest.Lodgings and dest.Lodgings2) are null. How do I restore the respective collections? If my class only has one collection of a particular type, I could do the following:
var lodgings = context.Lodgings.Where(l => l.Destination.DestinationId == destId).ToList();
I can see that the relationships are maintained in the database schema (Destination_DestinationId1 and Destination_DestinationId2) but I don't seem to be able to get to them.
Any suggestion would be appreciated.
In addition to using Include (as you've discovered) (which loads the related data from the db at the same time the destination is retrieved) you can also retreive the lodgings after the fact. So if you query for the destination and then you want the lodgings, that's possible. One way is called explicit loading where you will use a Load method. The other is with lazy loading, which requires that your classes be set up a particular way and just the mere mention of the Lodgings property will trigger the call to the database to retrieve them.
there's a great blog post on the Ef team blog about the various ways to load related data with DbContext : http://blogs.msdn.com/b/adonet/archive/2011/01/31/using-dbcontext-in-ef-feature-ctp5-part-6-loading-related-entities.aspx
hth
Julie
A little new to EF, so please bear with me if the answer to this is obvious. I'm doing a tutorial that uses EF, and two DbSets are defined like this:
public DbSet<BrokerageAccount> BrokerageAccounts { get; set; }
public DbSet<Customer> Customers { get; set; }
The customer class looks like this-- it's a POCO (some code cut for brevity):
public class Customer
{
public Customer()
{
BrokerageAccounts = new HashSet<BrokerageAccount>();
}
// Primitive properties
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
// Navigation properties
public ICollection<BrokerageAccount> BrokerageAccounts { get; set; }
}
}
The BrokerageAccount class is a POCO as well, very similar in design to Customer.
So far so good. The code I have a question about is below. There is an association made in the main program between Customer and BrokerageAccount that I don't follow. The code reads like this:
public Customer GetCustomer(string custId)
{
using (var context = DataContext)
{
return context.Customers
.Include("BrokerageAccounts").SingleOrDefault(c => c.CustomerCode == custId);
}
}
I can't figure out how the association/join is made between Customer and BrokerageAccount. I don't see any config or other files in my VS 2010 project that tells what associates the two DbSets, what foreign key column to use, etc.
Perhaps I'm missing something obvious or a mapping file of some sort, but just because Customer has an ICollection of BrokerageAccount along with a comment above that says "Navigation Properties", doesn't make it so. In EF, how are those associations established?
The normal way of setting up the navigation properties is to use the ModelBuilder, This gives you a fluent api to set up the associations, take a look at this for some in depth stuff about how you go about this.
http://xhalent.wordpress.com/2011/01/21/configuring-entity-framework-4-codefirst/
Entity framework will guess at what you meant if you dont set up the nav properties manually, in the above case it will probably set up your nav properties as expected as you only have a single 1-* relationship between customer and BrokerageAccount which appears to be named sensibly.
There is also an attribute method that you can use to set up the navigation properties.
Say I have Project and Task EF Code first classes
public class Project
{
public int ID { get; set; }
public string Name { get; set; }
public virtual ICollection<Task> Tasks { get; set; }
}
public class Task
{
public int ID { get; set; }
public string Name { get; set; }
public int ProjectId { get; set; }
public bool IsDeleted {get; set;}
public virtual Project Project { get; set; }
}
Say I have
public void SomeAction()
{
Project p= repository.GetById(1);
var tasks = p.Tasks;
//var tasks = p.Tasks.Where(t=>t.IsDeleted==false);
}
I would like that my Tasks property on the Project class will always perform that filter on IsDeleted and just return that subset ... to avoid having to write that condition all over the place...
Any recommendations?
Edit:
Im using EF Code First
Add a discriminator to your model in the OnModelCreating method
modelBuilder.Entity<TEntity>().Map(m => m.Requires("IsDeleted").HasValue(false));
Caveats
You can no longer load deleted items (unless you map IsDeleted true to another entity, then you may lose your automatic filtering)
The poco class cannot have the IsDeleted property (discriminators cannot be mapped)
because the IsDeleted cannot be mapped you need to run raw SQL to delete the entity in the first place.
EF Code first = NO WAY. Just one from long list of features which is available in EDMX and it is completely missing in code first. Mapped condition from EDMX does this but it is still problematic because it is hardcoded and cannot be changed (= you will never be able to load deleted entities even if you want to unless you use another EDMX). The solution would be implementation of global filters in EF but EF doesn't have anything like that despite the fact that old Linq-to-entities have them at least for relations (DataLoadOptions.AssociateWith).
This is much more painful in relations where you cannot use eager or lazy loading without loading deleted entities to your application as well and do filtering in your application's memory.
In the Model Designer, select your Task entity, and bring up the Mapping Details window. This should show you the database table your entity is mapped to, and all the columns. Just under where it says "Maps to [YourTable]" you should see an option <Add a Condition>. This should let you set a condition like what you're looking for.