This question already has answers here:
Linq relational table include
(2 answers)
Closed 3 years ago.
I have 2 entities Book and Borrow, if I fetch 1. a list of Book entities in one query then fetch 2. a list of Borrow entities in another, how can I use LINQ to build a list like this?
List of books:
Book.Borrows
Book.Borrows
Book.Borrows
Book.Borrows
So each Book has a list of it's borrows mapped to it from the 2nd query that fetches the Borrow list.
Navigation properties:
Class Borrow:
public virtual Book Book { get; set; }
Class Book:
public virtual List<Borrow> Borrows { get; set; }
No need to make 2 queries. You can just do that in a single one. You can refer both in a single query. That's the use of having navigation properties in the first place. So you can easily traverse the related objects too.
context.Books
.Include("Borrows") // If you have lazy loading
.Where(..some condition...)
.Select(b => new BookModel
{
BookName = b.Name,
Borrows = b.Borrows.Select(...select properties...).ToList()
}
Related
I am trying to query my postgresql database using Ef core and Ardalis Specification.
For the query I build I want to sort the results by using OrderBy with an aggregate of a property that is on a nested object.
The sorting I want is to sort the list of Clinics by the Clinic that has the most Reviews with high Grades. The grades are on a scale of 1-5.
So if a clinic has two reviews with Grade=5 it should come on top of a clinic that has 5 reviews with Grade=2 or Grade=4. To do this I have to calculate the mean value and then order by the highest
public class Clinic
{
public int Id { get; set; }
public ICollection<Review> Reviews {get; set;}
}
public class Review
{
public int Id { get; set; }
public decimal Grade {get; set;}
}
My query so far, which doesnt work as intended as it only gets the highest value. Can I insert a mean-value calculation here somehow?
public ClinicFilterPaginatedSpecification()
{
Query.OrderByDescending(x => x.Reviews.Max(x => x.Grade ));
}
Running the query:
var filterSpec = new ClinicFilterSpecification();
var itemsOnPage= await _clinicRepo.ListAsync(filterSpec);
As Ivan Stoev notes in his comment, you should be able to use the .Average() command:
public ClinicFilterPaginatedSpecification()
{
Query.OrderByDescending(clinic => clinic.Reviews.Average(review => review.Grade ));
}
Have you tried this and if so is it working or producing an error?
I should mention that this is not directly related to the Specification package, in the sense that the expression is not altered in any way. Whatever works on EF, should work through specs as well. We're passing the expression as it is.
Now the question is how EF would behave in this case when you need to aggregate some data from the collections. I think this is optimized in EF Core 5, and Ardalis' suggestion should work. Prior to EF Core 3, this scenario would have involved an explicit Join operation (not quite sure).
I have 2 projetcs PRJ1 and PRJ2 which uses their own databases DB1 and DB2. Each of these databases uses EF Code First Migration.
The PRJ1 is for managing stock of products (already exists since 4 years).
The PRJ2 is for orders (brand new project still in dev)
Now let's talk about the second project only. In my project PRJ2 I need to access data from the other database DB1. So I need to place orders for products.
Here is what I got so far for PRJ2
Note that I defined 2 different contexts.
// Context for accessing entities in DB1
public class DB1Context : DbContext
{
static DB1Context()
{
Database.SetInitializer<DB1Context>(null);
}
public DbSet<Product> Products { get; set; }
}
// Context for accessing entities in DB2
public class DB2Context : DbContext
{
static DB2Context()
{
Database.SetInitializer(new MigrateDatabaseToLatestVersion<DB2Context, DAL.Migrations.Configuration>());
}
public DbSet<Anything> Anythings { get; set; }
public DbSet<Order> Orders { get; set; }
}
What works: I can query data from DB1Context (Products) or DB2Context (Anythings).
What didn't works yet: Creating my Orders entity.
// My Orders entity
public class Orders
{
public int Id { get; set; }
public int ProductId { get; set; }
public int Quantity { get; set; }
public virtual Product Product { get; set; }
}
This entity located in DB2Context is referencing the Product entity which is part of DB1Context.
Now the problem is that as soon as I add this Entity inside my context DbSet<Order> I see that there is a migration waiting for Product. This migration is for creating Product in my DB2Context. That's not what I want. This entity already exists in DB1Context. It seems I cannot create this Order entity which is referencing Product from the other context.
Can you confirm this ? Did I missed something ? Otherwise what is the best alternative ?
I think you can't do that using Entity Framework.
That problem looks like a No-SQL database's problem. When you have multiple database like that you have to control all the CRUD for all databases. The ADO can't do it to you because you don't have data integrity.
A possible solution is you put your CRUD in your business logic layer.. or something like that.
Let suppose you have a OrderBll to control:
public class OrderBll
{
private DB1Context _DB1Context = new DB1Context();
private DB2Context _DB2Context = new DB2Context();
public List<Orders> GetOrders()
{
var orders = _DB2Context.Orders.Where(???).ToList();
var productIds = orders.Select(x => x.ProductId).Distinct().ToArray();
var products = _DB1Context.Products.Where(x => productIds.Contains(x.Id)).ToList(); // Optimize the load of all products in orders
// Set the product object in the order list
foreach( var order in orders )
{
order.Product = products.FirstOrDefault(x=>x.Id == order.ProductId);
}
return orders;
}
}
Remember you have to map the Product property in Orders like Ignore.
So, you have to set the Foreign Key by yourself and do all the constraints checks.
But, if someone else have a better solution to do that, I'll be glad to know.
Ideally, this is what you should aim for.
Put orders and products in the same database. You would then be able to create relations between orders and products. You would end up with a single EF context, and this would give you a solid solution.
If for whatever reasons you cannot put orders in the same database as products, then solutions are still available with their own limitations.
You may replicate the products table from DB1 to DB2, replication running every minute, for example. You can write your own replication component or use replication functionality of your database. If products can be deleted in DB2, replication could delete the products and orders in DB1, or just flagged deleted products. It is up to you to decide. If PRJ2 can update the products table, then replication has to be both ways. This gets more complicated. The EF context with this solution would contain relations from orders to products, and products to orders.
Another solution would be to keep in DB1 a "proxy" products table that contains the Id's of the products that are referenced in orders. Everywhere in your business logic, you decide whether you need to access the actual products table from DB1 or not. For example, when creating a new order, you would access products from DB1, and insert their Id's in the proxy table if not there. When displaying the products of an order, you would first retrieve the product Id's of the order from DB1, and then their full description from DB1. When updating the inventory of a product, as part of creating an order, you would access DB1, probably with a transaction that spans DB! and DB2.
Suppose you have one of the simplest text book models:
Product {
Categories
}
Where a product can be associated to 0 to many categories.
Suppose category looks similar to
Category {
int Id { get; set; }
string Name {get; set; }
}
Now I want to associate my product to (existing) categories with ids 1, 2, and 3. Is there anyway to create this association without loading categories 1, 2, and 3 into memory?
I know this would be possible with a single Category where my Product model had Category and CategoryId on it. Is there a similar convention for bonding multiple items?
To reiterate, my purpose is to avoid querying categories. I already have the identifiers. With direct sql I could easily establish these relationships by key only (in fact the association table is literally just the keys). I want to know if there's an "entity framework way" of doing this or whether direct sql is the only option.
You could create category instances with just the id and attach them to the context. Then you could add to the product, without having to pull the categories from the database.
For example:-
var category = new Category { Id = 1 };
db.Categories.Attach(category);
product.Categories.Add(category);
db.SaveChanges();
Is it possible to access the underlying DbContext (the DbContext that has populated this object/has this object in its cache/is tracking this object) from inside a model object, and if so, how?
The best answer I have found so far is this blog post which is five years old. Is it still the best solution available?
I’m using the latest version of Entity Framework if that matters.
Here's a sample to clarify my question:
I have a hierarchical tree. Let’s say it is categories that could have sub-categories. The model object would be something like this:
class Category
{
string CategoryId { get; set; }
string Name { get; set; }
virtual Category Parent { get; set; }
virtual ICollection<Category> Children { get; set; }
}
Now, if I want to access all descendants of a category (not just its immediate children) I can use a recursive query like this:
class Category
{
//...
IEnumerable<Category> Descendants
{
get
{
return Children.Union(Children.SelectMany(q => q.Descendants));
}
}
}
which works, but has bad performance (due to multiple database queries it needs to perform).
But suppose I have an optimized query that I can run to find descendent (maybe I store my primary key in a way that already contains path, or maybe I’m using SQL Server data type hierarchyid, etc.). How can I run such a query, which needs access to the whole table/database and not just the records available through model object’s navigational properties?
This can be either done by running a stored procedure/SQL command on the database, or a query like this:
class Category
{
//...
IEnumerable<Category> Descendants
{
get
{
// this won't work, because underlying DbContext is not available in this context!
return myDbContext.Categories.Where(q => q.CategoryId.StartsWith(this.CategoryId));
}
}
}
Is there a way at all to implement such a method?
I'm trying to put together a ViewModel that will have a list of users and each user will have a list of locations.
The User table and Location table are joined together through another table that holds each respective ID and some other information. This table is essentially a many to many join table.
I've tried a few different viewModel approaches and they we're severely lacking... What would be the best approach for displaying this type of information?
I assume that the issue is that you want to be able to access the collection by either User or Location. One approach could be to use ILookup<> classes. You'd start with the many-to-many collection and produce the lookups like this:
var lookupByUser = userLocations.ToLookup(ul => ul.User);
var lookupByLocation = userLocations.ToLookup(ul => ul.Location);
Update:
Per your description, it seems like you don't really need to have a full many-to-many relationship in your ViewModel. Rather, your ViewModel could have a structure like this:
public class YourViewModel
{
public IEnumerable<UserViewModel> Users { get; set; }
}
public class UserViewModel
{
// User-related stuff
public IEnumerable<LocationViewModel> Locations { get; set; }
}
If you wanted to avoid redundant LocationViewModel objects, you could pre-build a mapping between your Model and ViewModel objects:
var locationViewModels = myLocations.ToDictionary(
loc => loc, loc => CreateLocationViewModel(loc));
And then reuse these objects when populating your page's ViewModel.