I have a table TableA and child table TableB. I want to fetch all the parent table records,
but select child records which satisfy a condition. I am using include to get the child records.
Is there any direct way other than using select new?
LINQ to SQL has a LoadOptions that you can set on the context to do some powerful things. Most people point to the .LoadWith which eagerly loads child records. There's also an AssociateWith which specifies the filtering to apply on lazy child fetches. Both of them can take a lambda expression for sub child filtering. Here's an example:
var lo = new DataLoadOptions();
lo.AssociateWith<Customers>
(c => c.Orders.Where(o => !o.ShippedDate.HasValue));
this.LoadOptions=lo;
var query = from c in Customers
select c.Orders;
Note, this only works with LINQ to SQL. EF does not support this behavior at this time.
using (var context = new DbEntities()) {
foreach (var p in context.Parents) {
var childQuery = from c in p.Children
where c.whatever == something
select c;
// Do something with the items in childQuery, like add them to a List<Child>,
// or maybe a Dictionary<Parent,List<Child>>
}
}
Related
Below is a join using linq:
var query = from a in TableA
join b in TableB
on new { a.Id1, a.Id2 } equals new { b.Id1, b.Id2 }
select a;
var entities = query.ToList();
foreach(var item in entities)
{
foreach(var b in item.B)
{
var propValue = b.SomeProperty;
}
}
Assume that TableB have matching records for items in TableA.
But after executing query there was no value present in navigational property B of each A.
Also when I try to access it in inner foreach loop it do not send a request to DB to load those entities(lazy loading).
Am I missing something or this the way entity framework is supposed to work.
Because here in this case I expected that lazy loading will work and load the related entities when accessed.
Is there a way to materialize those navigational property while using the LINQ Query Syntax without loosing the ability to write clean sql query without sub queries.
You shouldn't join in LINQ to Entities. Instead use your Navigation Properties.
var query = from a in TableA.Include(a => a.B)
select a;
I am having at hierarchical table with the structure
ID, Name, FK_ID, Sortkey
Fetching the data in LINQ to SQL is straight forward:
var list = from ls in db.myTable
where ls.FK_ID == levelId
orderby ls.sortkey ascending
select ls;
And I can traverse down the tree by linking to the next levelId.
But what I can't figure out, if there is a way in LINQ, to check if there is any children
I could probably build a view, that added a flag to each record, but I would rather do this in LINQ, if possible.
What would even be the best practice for adding such a flag in SQL?
My idea on checking each record, is not the most performance friendly solution.
If you have set up the foreign key correctly, should you not have the 1 to Many mapping properties?
i.e. You could write
var listWithChildren = list.Where(l => l.Children.Any());
or going the other direction
var listWithParent = list.Where(l => l.FK_ID != null);
or using the query expression instead of fluent
var listWithChildren = from item in list
where item.Children.Any()
select item;
as you asked in your comments for a boolean flag, you could do
var updatedList = from item in list
select new
{
Item = item,
HasChildren = item.Children.Any()
};
I have a sql statement like this:
DECLARE #destinations table(destinationId int)
INSERT INTO #destinations
VALUES (414),(416)
SELECT *
FROM GroupOrder grp (NOLOCK)
JOIN DestinationGroupItem destItem (NOLOCK)
ON destItem.GroupOrderId = grp.GroupOrderId
JOIN #destinations dests
ON destItem.DestinationId = dests.destinationId
WHERE OrderId = 5662
I am using entity framework and I am having a hard time getting this query into Linq. (The only reason I wrote the query above was to help me conceptualize what I was looking for.)
I have an IQueryable of GroupOrder entities and a List of integers that are my destinations.
After looking at this I realize that I can probably just do two joins (like my SQL query) and get to what I want.
But it seems a bit odd to do that because a GroupOrder object already has a list of DestinationGroupItem objects on it.
I am a bit confused how to use the Navigation property on the GroupOrder when I have an IQueryable listing of GroupOrders.
Also, if possible, I would like to do this in one trip to the database. (I think I could do a few foreach loops to get this done, but it would not be as efficient as a single IQueryable run to the database.)
NOTE: I prefer fluent linq syntax over the query linq syntax. But beggars can't be choosers so I will take whatever I can get.
If you already have the DestinationGroupItem as a Navigation-property, then you already have your SQL-JOIN equivalent - example. Load the related entities with Include. Use List's Contains extension method to see if the desired DestinationId(s) is(are) hit:
var destinations = new List<int> { 414, 416 };
var query = from order in GroupOrder.Include(o => o.DestinationGroupItem) // this is the join via the navigation property
where order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId)
select order;
// OR
var query = dataContext.GroupOrder
.Include(o => o.DestinationGroupItem)
.Where(order => order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId));
I'm having trouble coming up with an efficient LINQ-to-SQL query. I am attempting to do something like this:
from x in Items
select new
{
Name = x.Name
TypeARelated = from r in x.Related
where r.Type == "A"
select r
}
As you might expect, it produces a single query from the "Items" table, with a left join on the "Related" table. Now if I add another few similar lines...
from x in Items
select new
{
Name = x.Name
TypeARelated = from r in x.Related
where r.Type == "A"
select r,
TypeBRelated = from r in x.Related
where r.Type == "B"
select r
}
The result is that a similar query to the first attempt is run, followed by an individual query to the "Related" table for each record in "Items". Is there a way to wrap this all up in a single query? What would be the cause of this? Thanks in advance for any help you can provide.
The above query if written directly in SQL would be written like so (pseudo-code):
SELECT
X.NAME AS NAME,
(CASE R.TYPE WHEN A THEN R ELSE NULL) AS TypeARelated,
(CASE R.TYPE WHEN B THEN R ELSE NULL) AS TypeBRelated
FROM Items AS X
JOIN Related AS R ON <some field>
However, linq-to-sql is not as efficient, from your explanation, it does one join, then goes to individually compare each record. A better way would be to use two linq queries similar to your first example, which would generate two SQL queries. Then use the result of the two linq queries and join them, which would not generate any SQL statement. This method would limit the number of queries executed in SQL to 2.
If the number of conditions i.e. r.Type == "A" etc., are going to increase over time, or different conditions are going to be added, you're better off using a stored procedure, which would be one SQL query at all times.
Hasanain
You can use eager loading to do a single join on the server to see if that helps. Give this a try.
using (MyDataContext context = new MyDataContext())
{
DataLoadOptions options = new DataLoadOptions();
options.LoadWith<Item>(i => i.Related);
context.LoadOptions = options;
// Do your query now.
}
Is there a way to use LoadWith but specify the fields that are returned?
For example, if I have two tables 1) Products 2) Categories
and do something like
DataLoadOptions dlo = new DataLoadOptions();
dlo.LoadWith<Products>(d => d.Categories);
db.LoadOptions = dlo;
MyDataContext db = new MyDataContext();
var result = from d in db.Products
select d;
when i check the query in profiler i see that ALL the rows from the Categories table are being returned. All I really need is the "Name" field.
I know I can rewrite the query using joins but I need to return the result set as a "Product" data type which is why I am using LoadWith.
No that's not possible with LoadWith.
You could try with a nested query in the projection, though that will be slow: 1 query per parent (so 1 query for the related category per product loaded).
You can use a projection, but you need to deal with anynomus types after that
select new {Order = order, ProductName = order.Product.Name,
CustomerName = order.Customer.Name,
OrderType = order.OrderType.Name } // etc