I have a data model where a 'Top' object has between 0 and N 'Sub' objects. In SQL this is achieved with a foreign key dbo.Sub.TopId
.
var query = context.Top
//.Include(t => t.Sub) Doesn't seem to do anything
.Select(t => new {
prop1 = t.C1,
prop2 = t.Sub.Select(s => new {
prop21 = s.C3 //C3 is a column in the table 'Sub'
})
//.ToArray() results in N + 1 queries
});
var res = query.ToArray();
In Entity Framework 6 (with lazy-loading off) this Linq query would be converted to a single SQL query. The result would be fully loaded, so res[0].prop2
would be an IEnumerable<SomeAnonymousType>
which is already filled.
When using EntityFrameworkCore (NuGet v1.1.0) however the sub-collection is not yet loaded and is of type:
System.Linq.Enumerable.WhereSelectEnumerableIterator<Microsoft.EntityFrameworkCore.Storage.ValueBuffer, <>f__AnonymousType1<string>>.
The data will not be loaded until you iterate over it, resulting in N + 1 queries. When i add .ToArray()
to the query (as shown in comments) the data gets fully loaded into var res
, using a SQL profiler however shows this isn't achieved in 1 SQL query anymore. For each 'Top' object a query on the 'Sub' table is executed.
First specifying .Include(t => t.Sub)
doesn't seem to change anything. The use of anonymous types doesn't seem to be the problem either, replacing the new { ... }
blocks with new MyPocoClass { ... }
doesn't change anything.
My question is: Is there a way to get behavior similar to EF6, where all data is loaded immediately?
Note: i realize that in this example the problem can be fixed by creating the anonymous objects in memory after executing the query like so:
var query2 = context.Top
.Include(t => t.Sub)
.ToArray()
.Select(t => new //... select what is needed, fill anonymous types
However this is just an example, i do actually need the creation of objects to be part of the Linq query as AutoMapper uses this to fill DTOs in my project
Update: Tested with the new EF Core 2.0, issue is stil present. (21-08-2017)
Issue is tracked on aspnet/EntityFrameworkCore
GitHub repo: Issue 4007
Update: A year later, this issue has been fixed in version 2.1.0-preview1-final
. (2018-03-01)
Update: EF version 2.1 has been released, it includes a fix. see my answer below. (2018-05-31)
The GitHub issue #4007 has been marked as closed-fixed
for milestone 2.1.0-preview1
. And now the 2.1 preview1 has been made available on NuGet as discussed in this .NET Blog post.
Version 2.1 proper is also released, install it with the following command:
Install-Package Microsoft.EntityFrameworkCore.SqlServer -Version 2.1.0
Then use .ToList()
on the nested .Select(x => ...)
to indicate the result should be fetched immediately. For my original question this looks like this:
var query = context.Top
.Select(t => new {
prop1 = t.C1,
prop2 = t.Sub.Select(s => new {
prop21 = s.C3
})
.ToList() // <-- Add this
});
var res = query.ToArray(); // Execute the Linq query
This results in 2 SQL queries being run on the database (instead of N + 1); First a plain SELECT
FROM
the 'Top' table and then a SELECT
FROM
the 'Sub' table with an INNER JOIN
FROM
the 'Top' table, based on Key-ForeignKey relation [Sub].[TopId] = [Top].[Id]
. The results of these queries are then combined in memory.
The result is exactly what you would expect and very similar to what EF6 would have returned: An array of anonymous type 'a
which has properties prop1
and prop2
where prop2
is a List of anonymous type 'b
which has a property prop21
. Most importantly all of this is fully loaded after the .ToArray()
call!