Just starting out with Entity Framework and am trying to work out how you would do something like this....
Say I have the following entities, Customers that have Orders that have OrderLineItems which are linked to Products. I would like to return the name of every customer with a count of the number of times they have ordered a particular product.
I have seen examples of using .Count() but these have always been for the first navigation property i.e. number of orders per customer.
Would appreciate some guidance here.
Something like this should work, where context is your DbContext instance.
It will return an IEnumerable<dynamic>, although obviously you could make a class to hold the results.
// The product to count
var productId = 12345;
context.Customers.Include("Orders.OrderLineItems.Products")
.Select(customer =>
new {
CustomerName = customer.Name,
ProductCount = customer.Orders
.SelectMany(o => o.OrderLineItems)
.SelectMany(i => i.Products.Where(p => p.Id = productId).Count()
});
The Include() extension method is useful, it will make sure that the resulting SQL query joins the relevant tables together - otherwise multiple queries would be executed for each customer (one to get orders, another for line items and a final one for products).
Related
The DbContext DbSet<T>.Load / DbSet<T>.LoadAsync methods return void and Task respectively: they execute queries and then add the loaded/instantiated entity objects into the DbContext's DbSet and update the navigation properties and reverse-navigation of already-loaded objects, but they don't return any information about what they loaded: there doesn't seem to be a way of getting the actual count of the number of rows that were loaded.
Which is surprising, considering that the SaveChanges / SaveChangesAsync method does return the number of rows affected by any DML statements it executes.
I know there's a workaround in that I could use ToList/ToListAsync instead and then use the List<T>.Count property, but that's defeating the point of using Load/LoadAsync.
For example, consider this two-step query operation:
async Task<PageViewModel> LoadOrdersAsync( Int32 customerId, Expression<Func<Order,Boolean>> predicate )
{
// Step 1:
List<Order> orders = await this.dbContext.Orders
.Where( o => o.CustomerId == customerId )
.Where( predicate )
.ToListAsync()
.ConfigureAwait(false);
// Step 1.5:
List<Int32> orderIds = orders.Select( o => o.OrderId ).ToList();
// Step 2:
await this.dbContext.OrderItems
.Where( i => orderIds.Contains( i.OrderId ) )
.LoadAsync()
.ConfigureAwait(false);
// Done!
return new PageViewModel( orders );
}
I want to get the quantity of OrderItem entities that were loaded in the second step, but as far as I know that isn't possible without using ToList/ToListAsync.
You’re right, there is no easy way to get the number of loaded entries of the Load. It is essentially the same as ToList without creating the list and adding the loaded elements to it. If you really don’t want to use ToList, one option is to access the DbContext.ChangeTracker and get the number of entries from that:
var entriesBefore = context.ChangeTracker.Entries().Count();
// load more entities
var loaded = context.ChangeTracker.Entries().Count() - entriesBefore;
Note, that this is not accurate when you include other, related entities in your query.
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'm developing a client app that uses breezejs and Entity Framework 6 on the back end. I've got a statement like this:
var country = 'Mexico';
var customers = EntityQuery.from('customers')
.where('country', '==', country)
.expand('order')
I want to use There may be hundreds of orders that each customer has made. For the purposes of performance, I only want to retrieve the latest order for each customer. This will be based on the created date for the order. In SQL, I could write something like this:
SELECT c.customerId, companyName, ContactName, City, Country, max(o.OrderDate) as LatestOrder FROM Customers c
inner join Orders o on c.CustomerID = o.CustomerID
group by c.customerId, companyName, ContactName, City, Country
If this was run against the northwind database, only the most recent order row is returned for each customer.
How can I write a similar query in breeze, so that it runs on the server side and therefore returns less data to the client. I know I could handle this all on the client but writing some javascript in a querysucceeded method that could be run by the client - but that's not the goal here.
thanks
For a case like this, you should create a special endpoint method that will perform your query.
Then you can use an Entity Framework query to do what you want, using the LINQ syntax.
Here are two Web API examples:
[HttpGet]
public IQueryable<Object> CustomersLatestOrderEntities()
{
// IQueryable<Object> containing Customer and Order entity
var entities = ContextProvider.Context.Customers.Select(c => new { Customer = c, LatestOrder = c.Orders.OrderByDescending(o => o.OrderDate).FirstOrDefault() });
return entities;
}
[HttpGet]
public IQueryable<Object> CustomersLatestOrderProjections()
{
// IQueryable<Object> containing Customer and Order entity
var entities = ContextProvider.Context.Customers.Select(c => new { Customer = c, LatestOrder = c.Orders.OrderByDescending(o => o.OrderDate).FirstOrDefault() });
// IQueryable<Object> containing just data fields, no entities
var projections = entities.Select(e => new { e.Customer.CustomerID, e.Customer.ContactName, e.LatestOrder.OrderDate });
return projections;
}
Note that you have a choice here. You can return actual entities, or you can return just some data fields. Which is right for you depends upon how you are going to use them on the client. If they are just for display in a
non-editable list, you can just return the plain data (CustomersLatestOrderProjections above). If they can potentially
be edited, then return the object containing the entities (CustomersLatestOrderEntities). Breeze will merge the entities
into its cache, even though they are contained inside this anonymous object.
Either way, because it returns IQueryable, you can use the Breeze filtering syntax from the client to further qualify the query.
var projectionQuery = breeze.EntityQuery.from("CustomersLatestOrderProjections")
.skip(20)
.take(10);
var entityQuery = breeze.EntityQuery.from("CustomersLatestOrderEntities")
.where('customer.countryName', 'startsWith', 'C');
.take(10);
This is my real world example.
I have 4 tables:
Person
Plan
Coverage
CoveredMembers
Each person can have many plans, each of those plans can have many coverages. Each of those coverages can have many CoveredMembers.
I need a query that will apply a filter on Plan.PlanType == 1 and CoveredMembers.TermDate == null.
This query should bring back any person who has a medical type plan that is not terminated.
This SQL statement would do just that:
SELECT Person.*, Plans.*, Coverages.*, CoveredMembers.*
FROM Person P
INNER JOIN Plan PL ON P.PersonID = PL.PersonID
INNER JOIN Coverage C on PL.PlanID = C.PlanID
INNER JOIN CoveredMember CM on C.CoverageID = CM.CoverageID
WHERE CM.TermDate = NULL AND PL.PlanType = 1
I have figured out how to do this using anonymous types, but I sometimes need to update the data and save back to the database - and anonymous types are read only.
I was given a solution that did work using JOIN but it only brought back the persons (albeit filtered the way I needed). I can then loop through each person:
foreach (var person in persons) {
foreach (var plan in person.Plans{
//do stuff
}
}
But wouldn't that make a db call for each iteration of the loop? I have 500 persons with 3 unterminated medical plans each, so it would call the db 1500 times?
This is why I want to bring the whole data tree from Persons to CoveredMembers back in one shot. Is this not possible?
I believe this is accomplished in two parts:
Your query to determine the people you wish to have returned based on your criteria as discussed in this question previously: Entity framework. Need help filtering results
Properly setting the navigation properties for entities you want brought together to be eagerly loaded: http://msdn.microsoft.com/en-us/data/jj574232.aspx
For example if your Person entity looks like:
public class Person {
public List<Plan> Plans {get; set;}
...
}
When returning data from the dbcontext you can also use explicit eager loading with the include option:
var people = context.People
.Include(p => p.Plans)
.ToList();
....
If these are nested - coverage is part of plan, etc (which it looks like, it goes something like):
var people = context.People
.Include(p => p.Plans.Select(pl=>pl.Coverage).Select(c=>c.CoveredMembers)))
.ToList();
....
I am making some assumptions about your data model here, and my code above probably needs a little tweaking.
EDIT:
I might need someone else to weigh in here, but I don't think you can add the where clause into an include like that (my example above leads you that way a bit by putting the include on the context object, instead return an IQueryable with your conditions set as solved in your first post (without a ToList() called on it) and then use the code you wrote above without the Where clauses:
From first post (you supplied different criteria in this one, but same concept)
var q = from q1 in dbContext.Parent
join q2 in dbContext.Children
on q1.key equals q2.fkey
join q3 in ........
where q4.col1 == 3000
select q1;
Then:
List<Person> people = q.Include(p => p.Plans
.Select(pl => pl.Coverages)
.Select(c => c.CoveredMembers).ToList();
Again, doing this without being able to troubleshoot - I am sure it would take me a few attempts to iron this one out too.
I'm quite a newbie in EF, so I'm sorry if my question has been answered before.. I just can't figure out the syntax..
I have two entities, Category & Product, where one category has many products.
I want to get all categories, with only their latest product (it has a date property named timestamp)
I have no idea how to do that. :-/
If possible I'd like to know the syntax of the two ways to write it, both the sql-like syntax, and the C# like syntax, e.g.:
ctx.Categories.Include("Products").ToList()
from c in ctx.Categories.Include("Products")
Thanks!
Here's the SQL-like way:
var categories =
from p in products
group p by p.Category into g
select new { Category = g.TheKey, LatestProduct = g.Max(p => p.TimeStamp) };
This is the Lambda-way (warning, untested):
var categories = products.GroupBy(p => p.Category)
.Select(g => new { Category = g.TheKey,
LatestProduct = g.Max(p => p.TimeStamp)});
A note on Categories.Include("Products"), you don't need this in your example. You use "Include" for eager-loading, so that for example if you had a list of Categories returned from EF, when you do Categories.Product you will get the associated product.
But all you require is a list of categories, and a single product for each one - which is already returned in the above LINQ query, so no need for Include.