Query: Error calling Count() after GroupJoin()

[john-luke-laue]

I'm getting a runtime error after doing a Count() on a GroupJoin()

Steps to reproduce

var query = _context.Users.GroupJoin(_context.Customers, u => u.Id, c => c.UserId, (u, c) => u);

//Error occurs on this line
int count = query.Count();

Exception message:
Expression of type 'System.Collections.Generic.IEnumerable'1[System.Collections.Generic.IEnumerable'1[MyClass]]' cannot be used for return type 'System.Collections.Generic.IEnumerable`1[System.Int32]'

Stack trace:
at System.Linq.Expressions.Expression.ValidateLambdaArgs(Type delegateType, Expression& body, ReadOnlyCollection'1 parameters) at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, String name, Boolean tailCall, IEnumerable'1 parameters) at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, Boolean tailCall, IEnumerable'1 parameters) at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, ParameterExpression[] parameters) at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateExecutorLambdaTResults at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel) at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](QueryModel queryModel) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass19_0'1.b__0() at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func'1 compiler) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQuery[TResult](Expression query) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query) at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression) at System.Linq.Queryable.Count[TSource](IQueryable'1 source) at MyProject.Services.User.UserService.SearchUsers(ContactSearchCriteriaDto cscd) in C:\Users\john-luke.laue\repositories\MyProject\src\Services\User\UserService.cs:line 59

Further technical details

EF Core version: 1.0.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10
IDE: Visual Studio 2015

[smitpatel]

public class Blog
{
    public int Id { get; set; }
    public string Title { get; set; }
    public List<Post> Posts { get; set; }
}

public class Post
{
    public int Id { get; set; }
    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

using (var db = new MyContext())
{
    // Run queries

    var query = db.Set<Blog>().GroupJoin(db.Set<Post>(), blog => blog.Id, post => post.BlogId,
        (blog, posts) => blog);

    var count = query.Count();
}

[smitpatel]

Transferring to @maumar ,
In this particular case, we have a group join but there is no selectmany afterwards. We apply count assuming it is safe but it generates query like this:

SELECT COUNT(*)
FROM [Blogs] AS [blog]
LEFT JOIN [Post7497] AS [post] ON [blog].[Id] = [post].[BlogId]

Which is incorrect since it would count number of rows instead of number of groups. We need to evaluate Count on client when there is group join without select many.

[maumar]

Fixed in 03a990c