Let's say I have a Person class and an Order class, with foreign keys in the DB. The EF model will mark Person with a List of Orders and Order with a Person instance.
If I want to set the Person for the Order, do I really have to do it with an instance of Person?
Is there not a slimmed down way to do so, say with just a PersonID ?
To assign Person entity to a Order without loading Person entity, you have to do something like this:
var db = new OneToManyEntities();
var Order = new Order { OrderId = 100, OrderName = "Order name" };
Order. PersonReference.EntityKey = new EntityKey("OneToManyEntities.Person ","PersonID",10);
db.AddToOrders(Order);
db.SaveChanges();
Puzzled's answer is correct for EF v1. It's a pain. If you don't mind the extra query, you can set the property succinctly:
int id = 1;
Order.Person = context.Persons.Where(x => x.PersonID == id).FirstOrDefault();
Entity Framework v4 will have "FK Associations", which is a fancy term for directly-settable foreign keys.
Related
Apparently, EF6 doesn't like objects that have multiple foreign key properties that use the same key value, but do not share the same reference. For example:
var user1 = new AppUser { Id = 1 };
var user2 = new AppUser { Id = 1 };
var address = new Address
{
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
};
When I attempt to insert this record, EF throws this exception:
Saving or accepting changes failed because more than one entity of type
'AppUser' have the same primary key value. [blah blah blah]
I've discovered that doing this resolves the issue:
var user1 = new AppUser { Id = 1 };
var user2 = user1; //same reference
I could write some helper code to normalize the references, but I'd rather EF just know they're the same object based on the ID alone.
As for why EF does this, one explanation could be that its trying to avoid doing multipe CRUD operations on the same object since separate instances of the same entity could contain different data. I'd like to be able to tell EF not to worry about that.
Update
So it's as I suspected per my last paragraph above. In absense of a means to tell EF not to do CRUD on either instance, I will just do this for now:
if (address.ModifiedBy.Id == address.CreatedBy.Id)
{
address.ModifiedBy = address.CreatedBy;
}
Works well enough so long as I am not trying to do CRUD on either.
Update2
I've previously resorted to doing this to prevent EF from validating otherwise-required null properties when all I need is the child entity's ID. However, it doesn't keep EF from going into a tizzy over separate instances with the same ID. If it's not going to do CRUD on either AppUser object, why does it care if the instances are different?
foreach (var o in new object[] { address.ModifiedBy, address.CreatedBy })
{
db.Entry(o).State = EntityState.Unchanged;
}
If you get AppUser from context, then you will not need to do anything, because Entity Framework will track entities:
var user1 = context.AppUsers.Find(1);
var user2 = context.AppUsers.Find(1);
var address = new Address
{
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
};
Now, they both will point to same objects and will not cause to conflict.
You can add two extra properties to have the Id for the main objects which is the AppUser, then you can use only one AppUser object and reference it for both the created and modified by properties.
CreatedById = user1.Id,
ModifiedById = user1.Id
Otherwise, your code will end up by saving two instances of AppUser with the same primary key.
Another approach is to set both the foreign key properties to only one AppUserobject
The explanation is that EF's change tracker is an identity map. I.e. a record in the database is mapped to one, and only one, CLR object.
This can be demonstrated easily by trying to attach two objects with the same key:
context.AppUsers.Attach(new AppUser { Id = 1 });
context.AppUsers.Attach(new AppUser { Id = 1 });
The second line will throw an exception:
Attaching an entity of type 'AppUser' failed because another entity of the same type already has the same primary key value.
This also happens if you assign
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
Somewhere in the process, user1 and user2 must be attached to the context, giving rise to the exception you get.
Apparently, you have a function that receives two Id values that can be different or identical. Admittedly, it would be very convenient if you could simply create two AppUser instances from these Ids, not having to worry about identical keys. Unfortunately, your solution ...
if (address.ModifiedBy.Id == address.CreatedBy.Id)
... is necessary. Solid enough, though.
Using entity framework, trying to get some help for a query (prefer method based syntax) for this typical use case:
There is a products table, like:
ownerId
productId
ProductCategoryId
productInfo, etc.
There is a typical product-category-mapping table, like:
somePrimaryKey
ownerId
categoryId
productId
sortOrder
This set up allows one product to be in multiple category, and has its own sort order in each category. Also, we have the "ownerId" in all tables since each owner can only see his own data.
Now, given a categtoryId and ownerId, we need to find all products of this category, sorted by the sortOrder.
Any way how we should write this?
Many Thanks!
You can try to use something along those lines :
// Instanciate your context.
// Do it the way you've already done it, it's here only for example.
DbContext bd = new DbContext();
// The query :
List<Products> listProducts = new List<Products>();
listProducts = db.Products.Where(p => (db.ProductsCategories.Where(pc => pc.CategoryID == categoryID && pc.OwnerID == ownerID).Select(pc => pc.ProductID).OrderBy(pr => pr.sortOrder).ToList()).Contains(p.ProductID)).ToList();
This way use the product-category mapping (categorieID and ownerID are the datas you inject and you keep the sorting.
I have an Entity. Mandate. Every mandate has a required:many relation to a Person (NavigationProperty). I use the DbContext API with (LazyLoadingEnabled, AutoDetectChangesEnabled, ValidateOnSaveEnabled, ProxyCreationEnabled)
Now I like to delete a Mandate entity. The mandate entities are loaded by another context with AsNoTracking().
message.Result.
ObserveOn(On<DataComposition>.Scheduler).
Where(r => r).
Subscribe(_ =>
{
using (var unit = UnitOfWork.Begin())
{
var mandate = this.SelectedItem.OriginalEntity;
this.mandateRepository.Attach(mandate);
// mandate.Person.ToString();
this.mandateRepository.Delete(mandate);
unit.Commit();
}
this.List.RemoveOnUi(this.SelectedItem);
});
Now during committing I get the following exception: Entities in 'CodeFirstContainer.Mandates' participate in the 'Mandate_Person' relationship. 0 related 'Mandate_Person_Target' were found. 1 'Mandate_Person_Target' is expected.
The delete works if I include the Person Property during the population/selection or if I visit the Property (lazyloading), but I DONT LIKE to materialize/hold many entities only for the deletion case and I DONT LIKE to trigger more than a single DELETE query to db!
The fact that, if you have the navigation property mandate.Person populated, the following SQL statement ...
delete [dbo].[Mandates]
where (([Id] = #0) and ([PersonId] = #1))
... is sent to the database, lets me think that the navigation property indeed must be populated with a person with the correct PersonId to delete the parent.
I have no idea why Entity Framework just doesn't send a delete statement with the primary key ...
delete [dbo].[Mandates]
where ([Id] = #0)
... as I had expected.
Edit
If the Mandate entity has a foreign key property PersonId for the Person navigation property, the expected SQL (the second above) is sent to the database. In this case the Person navigation property can be null and the value of the FK property PersonId doesn't matter.
Edit 2
If you don't want to introduce a FK property the way with the least DB-roundtrip-costs would probably be to fetch the person's Id and then create a dummy person with that key in memory:
// ...
var personId = context.Mandates
.Where(m => m.Id == mandate.Id)
.Select(m => m.Person.Id)
.Single();
mandate.Person = new Person { Id = personId };
this.mandateRepository.Attach(mandate);
this.mandateRepository.Delete(mandate);
// ...
I would like to add a record to a SQL Server table using the Entity Framework. My table's entity has foreign keys and so has navigational properties for those fields. When adding a new record/entity, how do I populate the foreign key fields since they don't appear as properties of the entity?
The easiest way is to do a query for the related entities and use the Navigation Properties:
i.e.
Product p = new Product{
ID = 5,
Name = "Bovril",
Category = ctx.Categories.First( c => c.ID == 5)
};
ctx.AddToProducts(p);
ctx.SaveChanges();
If you want to avoid the database query the easiest approach is probably to use a STUB entity i.e.
// this is a stub, a placeholder for the real entity
Category c = new Category {ID = 5};
// attach the stub to the context, similar to do a query
// but without talking to the DB
ctx.AttachTo("Categories", c);
Product p = new Product{
ID = 5,
Name = "Bovril",
Category = c
};
ctx.AddToProducts(p);
ctx.SaveChanges();
If you want more help on this stub technique check out this blog post on the topic.
I have a category table which has a foreign key to it self by a nullable parentId field. In Entity-Framework when a relation is created, the entity is generated without any relation fields. I mean, in my example when I created a parentId-Id relation in Category table, the generated Category Entity will have an int typed Id property and a Category typed ParentCategory property and no ParentId property. And this makes my queries harder.
So, I have a trouble when I want to select the subcategories of a category. I use the method below for that;
public IEnumerable<ICategory> GetSubCategories(long? categoryId)
{
var subCategories = this.Repository.Category.Where(c => c.ParentCategory.Id == categoryId)
.ToList().Cast<ICategory>();
return subCategories;
}
But this does not work, when I want to select the root categories. What is the way of doing this?
By the way, I wonder if there is a way to generate entities like in Linq to Sql, with an int typed Id property, an int typed ParentId and a Category typed ParentCategory property.
To select the root categories:
var rootCategories = this.Repository.Category.Where(c => c.ParentCategory == null).ToList().Cast<ICategory>();
To generalize the code to work with both sub & root categories:
var rootCategories = this.Repository.Category.Where(c => categoryId == null ? c.ParentCategory == null : c => c.ParentCategory.Id == categoryId).ToList().Cast<ICategory>();