I'm a bit confused by conflicting examples of one-to-many model relationships using EF that I'm seeing online.
One video I watched setup a relationship between tables like so:
public class CustomerType
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
public int CustomerTypeId { get; set; }
public CustomerType CustomerType { get; set; }
}
So a Customer can only have one CustomerType, but a CustomerType can be used by many Customers. This code works fine, I can fetch a Customer's CustomerType with LINQ using Include.
Now I'm looking at another resource showing the same kind of relationship:
public partial class Standard
{
public int StandardId { get; set; }
public string StandardName { get; set; }
public virtual ICollection<Teacher> Teachers { get; set; }
}
public partial class Teacher
{
public int TeacherId { get; set; }
public string TeacherName { get; set; }
public Nullable<int> StandardId { get; set; }
public virtual Standard Standard { get; set; }
}
This looks almost the same, except:
In this example, the Standard class (equivalent to my CustomerType) has a navigation property back to a collection of Teachers, which my first example does not have. Is this just convenient to have if I want to get a list of all teachers for a given standard, or is it necessary to properly set up the relationship?
The properties in the second example are marked virtual and the first are not -- it seems that best practice is to make nav properties virtual, but is there a reason you wouldn't want to do that?
If it matters, I'm using MVC5 and EF6 and I just want to know if one example is right and one is wrong, or just two styles of getting to the same place.
Thanks!
The navigational properties are to make queries easier for the programmer. Your examples are basically the same with the difference that in Standard you can access Teachers through query while in CustomerType you can not access Customers with this CustomerType because you do not have it as a navigational property. Nevertheless, you can always include List<Customer> Customers in Customer Type.
Also it is better to add virtual to your navigational property for the sake of lazy loading.
MSDN
It depends on your needs, if you will never have to get the navigation property and just need a foreign key for sake of data integrity then you can simply add an integer and mark it as a foreign key. ex: instead of having a CustomerType instance, you can simply have a CustomerTypeId and that is it.
As for the virtual keyword, you can add it if you want to have a lazy loading enabled in your DBContext, cause EF generates proxy classes that inherits from your model classes and it overrides the virtual properties to add the needed logic to lazy load the navigation properties.
If you have a lazy loading disabled, then no need to mark any property as virtual
Related
I am writing the Entity Framework models Code-First with no existing database. On my objects, I have declared my one-to-many foreign relationships both explicitly and as lazy-loading navigation properties like this:
UserRecord class
[Key]
public long ID { get; set; }
public virtual List<WorkItemRecord> WorkItemsAuthored { get; set; } // authored work items
WorkItemRecord class:
[Key]
public long ID { get; set; }
public long AuthorID { get; set; } // user ID of the author
public virtual UserRecord Author { get; set; } // navigation lazy-loaded property
The idea behind maintaining the foreign key both as an ID as well as a navigation property in the WorkItemRecord class is that in cases where I just need the actual underlying author's user ID, I can reference that directly without having to call the property and incur another DB lookup.
The problem is, when EF creates the database schema, it doesn't tie them together. It creates separate columns: AuthorIDand UserRecord_ID
I thought at first maybe this was because my property was Author and not User. However, even when I explicitly specified it using the property attribute...
[ForeignKey("Author")]
public long AuthorID { get; set; }
public virtual UserRecord Author { get; set; }
... I still end up with those same two columns in the generated schema.
Also tried putting the decorator on the other property...
public long AuthorID { get; set; }
[ForeignKey("AuthorID")]
public virtual UserRecord Author { get; set; }
... and still get the same result.
I'd like to avoid the Fluent API if possible, but I'm open to it if I can't get any other solutions.
Any ideas?
Thanks!
UPDATE
Thanks for all the help! Fluent API worked fine, but fixing the data annotation with an inverse property decorator proved much easier. Final solution:
UserRecord class:
public long ID { get; set; }
public virtual List<WorkItemRecord> WorkItemsAuthored { get; set; }
WorkItemRecord class:
public long ID { get; set; }
[ForeignKey("Author")]
public long AuthorUserID { get; set; }
[InverseProperty("WorkItemsAuthored")]
public virtual UserRecord Author { get; set; }
Microsoft also has an article talking about this specific issue:
https://msdn.microsoft.com/en-us/data/jj591583.aspx#Relationships
In you your DbContext class add this to your OnModelCreating method.
modelBuilder.Entity<Book>() //Guessing at your class name
.HasRequired(e => e.Author)
.WithMany(e => e.Books)
.HasForeignKey(e => e.AuthorID);
This will enforce the constraint.
The reference property should be enough for EF to recognize the one-to-many relationship but do you have a navigation property in the UserRecord class as well?
public class UserRecord
{
/* other properties */
public virtual List<WorkItemRecord> WorkItemsAuthored { get; set; }
}
Maybe there is a minor detail which causes EF not to recognize the foreign key.
UPDATE:
Try InverseProperty in the UserRecord class as below:
[InverseProperty("WorkItemsAuthored")]
public virtual UserRecord Author { get; set; } // navigation lazy-loaded property
From the "Programming Entity Framework: Code First" written by Julia Lerman and Rowan Miller:
... you may run into a scenario where there are multiple relationships between entities. In these cases, Code First won’t be able to work out which navigation properties match up.
I assume that the UserRecord class has more than one List<WorkItemRecord>.
I got a problem on Foreign Keys.
What I want is that for every Video Model there is a Profile Model link to it
An example of that would look like this:
public class VideoModels
{
[Required]
public int Id { get; set; }
public ProfileModels Profile { get; set; }
}
While in the ProfileModels
public class ProfileModels
{
[Required]
public int Id { get; set; }
public string Title { get; set; }
}
Which technically should not have any connection with VideoModels for the VideoModels is dependent with ProfileModels and there can be 0 to many relationship.
So after that I tested it, my VideoModels has a Profile_Id parameter which is a foreign key from ProfileModels
But after creating an object and retrieving it, it returns NULL but when I check the database there is an existing Foreign Key Id.
Please help, stuck here for hours now
Profile would be null here due to LazyLoading. This allows us to have an object with many different objects that are only loaded when needed rather than loaded when a parent object is loaded. This is especially helpful if you have something like a Comments property as part of the user. Without LazyLoading the comments collection would be loaded whenever you wanted to grab just the VideoModels.ID. A downside to LazyLoading is that each navigation performed requires a separate query to the data source.
To allow for LazyLoading you need to mark related object properties as virtual.
In addition you must supply a navigation property in your context:
public class Database : DbContext {
public DbSet<VideoModels> People { get; set; }
public DbSet<ProfileModels> PersonDetails { get; set; }
}
Or
If you want to use your current class modeling, you can achieve this by disabling LazyLoading.
this.Configuration.LazyLoadingEnabled = false;
Nevermind, I just forgot to make the foreign key propery as virtual.
public class VideoModels
{
[Required]
public int Id { get; set; }
public virtual ProfileModels Profile { get; set; }
}
Using entity framework 6 and mvc 5, assuming I have the following models:
class Employee
{
public int EmployeeID { get; set; }
public String Name { get; set; }
public Department Department { get; set; }
}
class Department
{
public int DepartmentID { get; set; }
public String DepartmentName { get; set; }
public int FloorNumber { get; set; }
}
In my DbContext class where the DbSet goes, do I do DbSet<Employee> only or do I also have to do DbSet<Department>? Right now I'm only doing DbSet<Employee> because the way I understand it, the main model is Employee which has a complex type Department inside, so theoretically if Employee loads then Department should load passively (load as in create a table)?
Currently, I have Department as a virtual field for lazy loading because when I try to access it without the virtual attribute I get a null pointer exception.
Also, my model is a little more complex than this with multiple complex objects, do each of these need their own DbSet?
Thank you
According to the documentation (emphasis is mine):
Type Discovery
<snip>
In the following example, there is only one DbSet property defined on
the SchoolEntities class (Departments). Code First uses this property
to discover and pull in any referenced types.
However, if you ever wish to manipulate lists of Employees independent from your Department, then you should include it.
I'm building a fairly simple MVC project and still getting my head around where to use navigation properties and foreign keys with code first.
This is the main model class:
public class GroceryItem
{
public int ID { get; set; }
public string Name { get; set; }
public GroceryCategory Category { get; set; }
public QualityProfile Quality { get; set; }
public GroceryStore BestStore { get; set; }
public double BestPrice { get; set; }
public double LastSeenPrice { get; set; }
//Navigation Properties
public virtual ICollection<GroceryItem> SimilarItems { get; set; }
}
and these are the relating classes:
public class GroceryStore
{
public int ID { get; set; }
public string Name { get; set; }
public string Address { get; set; }
public Uri Website { get; set; }
}
public class QualityProfile
{
public int ID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
/// <summary>
/// Rank out of 1-10, 10 being the best
/// </summary>
public byte Ranking { get; set; }
}
public class GroceryCategory
{
public int ID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
}
Which brings me to my question, is the navigation property of SimilarItems I have in the GroceryItem class sufficient to represent a list of multiple grocery items or does this not work as it is referring to itself?
Additionally...do the Category, Quality and BestStore properties require ID properties to represent a foreign key inside of the GroceryItem class (e.g. CategoryID), or is the way I have this represented OK?
----EDIT----
--Refactored Code--
I've re-factored my model based on the suggestions below, which I think better accommodates the suggestions you've made (yes a 2nd time), realised my model was a little flawed and extracted out the price component into a separate purchases Model.
public class GroceryItem
{
public int ID { get; set; }
public string Name { get; set; }
[ForeignKey("Category")]
public int CategoryID { get; set; }
[ForeignKey("Quality")]
public int QualityID { get; set; }
//Navigation Properties
public virtual QualityProfile Quality { get; set; }
public virtual GroceryCategory Category { get; set; }
}
However the last thing I'm uncertain about which is on topic to this post, is if I have a collection as a part of the model (one that does not reference itself like in the first example), can I just represent that with a navigation property or does an extra step need to be taken?
Ie. If I was to allow multiple different categories on a GroceryItem, instead of looking like this:
[ForeignKey("Category")]
public int CategoryID { get; set; }
public virtual GroceryCategory Category { get; set; }
it would look like this:
public virtual ICollection<GroceryCategory> Categories { get; set; }
The best answer to your question(s) is, "It depends". Navigation properties are one way of informing Entity Framework that there's a relationship between entities. By convention, if you have a navigation property such as:
public Category Category { get; set; }
Entity Framework will create a column on the table named in the form of [RelatedPropertyName]_[RelatedPK]. Given your classes, the property above would cause a column named Category_ID. There's nothing more you need to do make it work. The relationship will automatically be handled by EF.
However, doing it this way, you won't have access to this foreign key property. It's not exposed in the public API of your entity. Often, especially when selecting related items from a select list and similar such scenarios, this becomes problematic, as you must store the selected value some place else, usually a property on a view model, and then use this to query the related thing from the database before setting it on the entity it belongs to and finally saving the entity. Whereas, with an actual foreign key property, you can simply post directly back to this and Entity Framework will automatically wire up the related entity. As a result, I tend to always follow the following pattern with my navigation properties:
public int FooId { get; set; }
public virtual Foo Foo { get; set; }
In most scenarios, Entity Framework will automatically connect those two, such that FooId will hold the foreign key relationship for the Foo navigation property. However, occasionally, EF will trip up and try to create the implicit foreign key behind the scenes, still, but you can correct that behavior by explicitly telling EF that this is the foreign key:
[ForeignKey("Foo")]
public int FooId { get; set; }
Roughly the same applies with collection navigation properties. EF will see this as an indication that there's a one-to-many relationship in play and add the implicit foreign key on the opposite entity. Given your collection:
public virtual ICollection<GroceryItem> SimilarItems { get; set; }
The opposite entity is actually the same entity, which presents an interesting use case. Typically, EF would handle this by assuming there's a one-to-many relationship. You'd end up with a column named GroceryItem_ID on your dbo.GroceryItems table. Here, though, you would not only have no access to the foreign key directly, but you also have no public API for accessing the parent GroceryItem either. That may not be a problem, but it's something to be aware of. The only way you'd be able to manage the relationship is through the collection on the parent, not through a child item in that collection.
However, since this is self-referential and you have not specify a foreign key or instance navigation property, all EF will see is a collection on both sides of the relationship, so my guess is that you'll actually end up with an M2M with an intermediary table. I can't test that theory out myself at the moment, and I haven't tried this particular scenario myself previously.
To create a true one-to-many, you would need to create another navigation property similar to:
public virtual GroceryItem ParentGroceryItem { get; set; }
And, even, then, I don't think EF will get the point without a little Fluent configuration:
HasMany(m => m.SimilarItems).WithOptional(m => m.ParentGroceryItem);
You could also use WithRequired in other scenarios instead of WithOptional, which would obviously make the relationship a required one, but since this is self-referential, it's impossible to have it required, because there will have to be at least one root node with no parent.
I have entities:
[Table("Currency")]
public class Currency
{
[Key]
public int CurrencyId { get; set; }
public string Name { get; set; }
public char Symbol { get; set; }
}
[Table("Invoice")]
public class Invoice
{
[Key]
public int InvoiceId { get; set; }
public int CurrencyId { get; set; }
public int Amount { get; set; }
// public virtual ICollection<Currency> CurrencyList { get; set; }
}
I need every invoice to contain all possible currencies that are in the database. If I just set navigation property as in the commented line, then the system tries to implicitly create its own foreign key constraint that doesnt actually exists and then gives an error.
Is there a way to make that navigational prop with no FK?
I don't believe a navigation property is what you're looking for. this msdn article discusses navigation properties more in depth, http://msdn.microsoft.com/en-us/library/vstudio/bb738520(v=vs.100).aspx
Every object can have a navigation property for every relationship in which it participates.
this sentence specifically and the two sections Modifying Relationships and Navigating Relationships point more to the fact that these properties are specifically for EF relationships, if there is no relationship between the two objects there likely shouldn't be a navigation property.
#carlosfiguira has a good idea about a helper class, which i agree with. but hearing your issues with adding helper code to your CodeFirst models.
My suggestion would be to create a composite model that has an invoice property and the list of currencies. I've done this before especially in Web patterns, that type of complex object might be called a ViewModel depending on use case.