Translate nested query SQL to EF - entity-framework

Question

I have this table structure:
Classic many to many relationship. I want to get all the orders for products belonging to the category for a small number of products I provide. It may be easier to show the SQL that does exactly what I want:
select o.*
from [Order] o join Product p2 on o.FKCatalogNumber=p2.CatalogNumber
where p2.FKCategoryId IN
(select c.Id
from Category c join Product p1 on p1.FKCategoryId=c.Id
where p1.CatalogNumber in ('0001', '0002')
This example gives me all the orders belonging to the categories that catalog #'s 0001 and 0002 are in.
But I am unable to wrap my head around the equivalent EF syntax for this query. I'm embarrassed to say I spent half the day on this. I bet it's easy for someone out there.
I came up with this but it's not working (and probably not even close):
string[] catNumbers = {"0001", "0002"};
var orders = ctx.Categories
.SelectMany(c => c.Products, (c, p) => new {c, p})
.Where(#t => catNumbers.Contains(#t.p.CatalogNumber))
.Select(#t => #t.p.Orders)
.ToList();

Answer 1

You can use query syntax (which looks very similar to SQL) in LINQ, so if you're more comfortable with SQL then you may prefer to write your query like this:
string[] catNumbers = {"0001", "0002"};
var orders = from o in ctx.Orders
join p2 in ctx.Products on o.FKCatalogNumber equals p2.CatalogNumber
where
(
from c in ctx.Categories
join p1 in ctx.Products on c.ID equals p1.FKCategoryId
where catNumbers.Contains(p1.CatalogNumber)
select c.ID
).Contains(p2.FKCategoryId)
select o;
As you can see, it's actually just your SQL query rearranged slightly, but it compiles as C#.
Note that:
the [Order] o syntax for referencing tables is replaced by o in ctx.Orders
LINQ enforces which way round you do the join condition so I had to flip your on o.FKCatalogNumber=p2.CatalogNumber to be on o.FKCatalogNumber equals p2.CatalogNumber
instead of your where p2.FKCategoryId IN (...), the equivalent c# is (...).Contains(p2.FKCategoryId)
the select comes last, not first
but those are the only major changes. Otherwise, it's written just like SQL.
I'd also draw your attention to a distinction regarding this comment:
the equivalent EF syntax for this query
The syntax here isn't specific to EF, but is just LINQ - Language Integrated Querying. It has two flavours: query syntax (sometimes called declarative) and method syntax (sometimes called fluent). LINQ works on just about any collection that implements IEnumerable or IQueryable, including EF's DbSet.
For more info on the different ways of querying, this MSDN page is a decent place to start. There's also this handy reference table showing the equivalent query syntax for each method-syntax operator, where applicable.

Answer 2

You can still nest queries in EF. The following looks like it works for me:
string[] catNumbers = {"0001", "0002"};
var orders = ctx.Orders
.Where(o => ctx.Products
.Where(p => catNumbers.Contains(p.CatalogNumber))
.Select(p => p.CategoryId)
.Contains(o.Product.CategoryId)
);
This produces the following SQL:
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[CatalogNumber] AS [CatalogNumber]
FROM [dbo].[Orders] AS [Extent1]
WHERE EXISTS (SELECT
1 AS [C1]
FROM [dbo].[Products] AS [Extent2]
INNER JOIN [dbo].[Products] AS [Extent3] ON [Extent2].[CategoryId] = [Extent3].[CategoryId]
WHERE ([Extent1].[CatalogNumber] = [Extent3].[CatalogNumber]) AND ([Extent2].[CatalogNumber] IN (N'0001', N'0002'))
)