i'm struggling with LLBLGEN and i guess ORM's in general.
i have created an entity, lets use a library example to explain:
i want to display a book object and also return a list of users who have loaned the book.
so i need to return the book object which contains a list of users.
DTO Book::
int bookId,
string bookName
additionally i wish to return with my book a collection of users who have loaned the book:
List<user> Loans
loans table might look like this:
int id
int userid
int bookid
currently my loans entity has now created this:
DTO Loans
int id
User user // user entity
Book book // book entity
im struggling to understand how this example would work in llblgen. can anyone assist with guidance or point me in the way of a tutorial?
at the moment, when i come up to update my model Book with a new loan associated to a book, im getting stackoverflow errors. i assume this is creating some sort of loop when attempting to update my Book object.
thanks
i noticed when running a profiler on SQL that the sql statement didnt include any join statements onto my relationship entities.
this was because my domain query didnt include prefetch items for my relationships, i added the following to my query:
var query = new DomainSearch<T>
{
SearchText = searchText,
PrefetchItems =
{
new Prefetch(typeof(Users))
}
};
To make sure, you are looking for a list of User entities that have loaned a particular Book entity. Is this the correct use case, or are you looking for a list of User entities that have borrowed the particular book?
Regardless, LLBLGen's support for these cases is great with referencing relationships between entities and using related entities quickly and easily.
Presuming you're looking up a book by unique BookId/ISBN/etc....
// Get the specific book entity
BookEntity book = new BookEntity(bookId);
foreach(UserEntity user in book.users)
{
// do stuff with list of users
}
It's just that simple assuming you've defined your relationships between entities.
Related
I have 2 entities.
entity 1 - People
entity 2 - books
People entity has a property which is an array of string names of their favorite books.
I need to create a relationship that somehow maps the favorite book of a person to the corresponding book entity object(s).
I am not sure how to do this.
So far I have started by creating a relationship in core data model for people by setting destination to "books" and then for the books entity creating a relationship by setting destination to "people".
I don't see or understand how this will automatically pick out each of the person's favorite books...at the end of the day they are both seperate objects. How will the people class know that for a specific people instance that this, this and this book are that person's favorite?
Person attribute as array of string names of books -- very bad idea!
You need a to-many relationship with the Book entity. That's it.
Person <------------>> Book
Then, to get an array of book titles for a particular person:
(person.books as! Set<Book>).map { $0.title }
The person can have an additional to-one relationship (e.g. called favoriteBook) to one of the books.
I have a table named product having product id, product bill id and.. the bill id is passed to my controller as a parameter.
I can use the entities from framework and find all rows with product id using
db.tbl_product.Find(product id).
But now i need to find all transactions using bill id. How do I do that??
Assuming tbl_product is a DbSet<Product> or something similar, you should be able to use LINQ to query the DbSet. To find a single item with a specific BillId property value, you would do something like this:
var product = db.tbl_product.FirstOrDefault(p => p.BillId == billId);
If there were multiple products with the same BillId, you could do the following:
var products = db.tbl_product.Where(p => p.BillId == billId);
It largely depends on the schema of the table and how you're using Entity Framework. I would highly recommend reading a book or tutorial on Entity Framework. There are lots of them out there, for example: Entity Framework Tutorial.
I've just started working with Web API this week, and I'm struggling with something which I think should be quite simple, but haven't been able to find the answer for yet. Perhaps I'm searching using the wrong terms.
One of the calls to the API passes through a GUID. I need to create a new entity (using Entity Framework) and set one of the relations to this newly passed in GUID. This GUID is the ID of a different entity in the database.
I'm struggling to attach the entity via the relation without fetching the whole entity too.
For example,
public void DoWork(IList<Guid> userGuids)
{
Order order = new Order() // This is an entity
{
CreateDate = DateTime.Now,
CreatedBy = "Me",
Items = (from i in this.Model.Items
where i.Id == userGuid
select i).ToList<Item>();
}
Model.Orders.Add(order);
Model.SaveAll();
}
In the above, I have to do a database call to attach the Item entities to the Order. Is there not a way around this? Seems very redundant to retrieve the whole entity objects when I only require their IDs (which I already have anyway!)!
One solution is stub entities as asked here: Create new EF object with foreign key reference without loading whole rereference object
Link to the source blog referenced: http://blogs.msdn.com/b/alexj/archive/2009/06/19/tip-26-how-to-avoid-database-queries-using-stub-entities.aspx
Snip from the blog - to be applied to your situation:
Category category = new Category { ID = 5};
ctx.AttachTo(“Categories”,category);
Product product = new Product {
Name = “Bovril”,
Category = category
};
ctx.AddToProducts(product);
ctx.SaveChanges();
This way (in the example) the Product is saved without ever loading the Category object.
OK, I have 3 tables, call them:
Person
PersonID
Name
Store
StoreID
Name
PersonStore
PersonID
StoreID
Now, I have a form which allows you to add stores to a person. However, I am getting the store ID back from the form. I don't really want to do a query to get the store object from Entity Framework. I just want to add to the table using the StoreID and the Person object which I have.
By default in EF this join table won't appear as an entity instead you'll get a many to many relationship which will show up as two navigation properties
i.e.
Person.Stores
Store.People
If you want to build a many to many relationship without retrieving the entities then attaching stub entities is the best way.
var person = // you already have the person
var store = new Store{StoreID = 5} // you know the storeID
ctx.AttachTo("Stores", store);
ctx.AttachTo("People", person); // assuming the person isn't already attached
person.Stores.Add(store);
ctx.SaveChanges();
The only problem with this code is it will fail if the relationship already exists, so you need to be sure you are creating a new relationship
For more on using Stub entities like this check out my post.
Hope this helps.
Alex
Edit from OP:
Since I am using EF4, I used the following code to remove the string from the attach (thanks to tip 13 from the link).
var person = // you already have the person
var store = new Store{StoreID = 5} // you know the storeID
ctx.Stores.Attach(store);
person.Stores.Add(store);
ctx.SaveChanges();
Suppose I have
table Person
table Employee, which inherits Person.
I want to get a list of Person, regardless if this Person is Employee or not. How do you get entity framework to do that without joining the Employee table? C# is what I'm using. Thanks.
You need to make sure that you don't implicitly use any information from Employee.
If you do this:
var people = Context.People.ToList();
... Then me Entity Framework will initialize new instances of type Employee for those people who happen to be employees. This is because when you store an Employee in the database, you generally expect to get an Employee back when you select the same item.
Nor, for that matter, can you tell the Entity Framework to give you a Person when the stored value is an Employee. The Entity Framework will never give you back an incorrect entity type.
However, if you do just want to read the one table, there is a way to do that: Select the data from Person into a non-entity types, such as an anonymous type:
var people = Context.People.Select(p => new { Name = p.Name }).ToList();