Multiple queries being generated when entire element selected

[meyerovb]

EF is producing multiple queries (n+1) instead of a single query with a subquery when the selection contains the entire element instead of just part of it.

Steps to reproduce

Set up a project as per https://docs.microsoft.com/en-us/ef/core/get-started/aspnetcore/new-db?tabs=visual-studio

context.Blogs.Select(a => new { a.Url, a.Posts.Count }).ToList();

runs this

SELECT [a].[Url], (
    SELECT COUNT(*)
    FROM [Posts] AS [p]
    WHERE [a].[BlogId] = [p].[BlogId]
) AS [Count]
FROM [Blogs] AS [a]

context.Blogs.Select(a => new { a, a.Posts.Count }).ToList();

runs this

SELECT [a].[BlogId], [a].[Url]
FROM [Blogs] AS [a]

exec sp_executesql N'SELECT COUNT(*)
FROM [Posts] AS [p0]
WHERE @_outer_BlogId = [p0].[BlogId]',N'@_outer_BlogId int',@_outer_BlogId=2

Further technical details

EF Core version: 2.2
Database Provider: mssqllocaldb
Operating system: Win10
IDE: (e.g. Visual Studio 2017 15.9.4)

[ajcvickers]

@smitpatel to find dupe.

[smitpatel]

Duplicate of #11186