Is it possible to use Skip and Take whilst calculating an Average?

Question

I am trying to create query using LINQ to Entities that calculates an average of a rating for a product. I project the average for each product to an anonymous object, before ordering the ratings. I then want to take only a portion of the results.

The query is below:

var query = Items.GroupBy(review => review.Product)
            .Select(group => new
            {
                Product = group.Key,
                Rating = group.Average(review => review.Rating)
            })
            .OrderByDescending(pr => pr.Rating)
            .Skip(itemsToSkip)
            .Take(count)
            .ToList();

The query that is generated looks like the following:

{SELECT 
1 AS [C1], 
[GroupBy1].[K1] AS [ProductId], 
[GroupBy1].[K2] AS [TotalImages], 
[GroupBy1].[K3] AS [TotalVideos], 
[GroupBy1].[K4] AS [TotalAudioRecordings], 
[GroupBy1].[K5] AS [Name], 
[GroupBy1].[K6] AS [DiscountedPrice], 
[GroupBy1].[K7] AS [Status], 
[GroupBy1].[A1] AS [C2]
FROM ( SELECT 
    [Extent2].[ProductId] AS [K1], 
    [Extent2].[TotalImages] AS [K2], 
    [Extent2].[TotalVideos] AS [K3], 
    [Extent2].[TotalAudioRecordings] AS [K4], 
    [Extent2].[Name] AS [K5], 
    [Extent2].[DiscountedPrice] AS [K6], 
    [Extent2].[Status] AS [K7], 
    AVG([Extent1].[Rating]) AS [A1]
    FROM  [dbo].[ProductReviews] AS [Extent1]
    LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[Product_ProductId] = [Extent2].[ProductId]
    GROUP BY [Extent2].[ProductId], [Extent2].[TotalImages], [Extent2].[TotalVideos], [Extent2].[TotalAudioRecordings], [Extent2].[Name], [Extent2].[DiscountedPrice], [Extent2].[Status]
)  AS [GroupBy1]}

The query doesn't work to skip any items, or to take only a few results. Rather it returns a calculation of the average for every product in the database.

Is it possible to utilise Skip and Take, as I am attempting to do? If so, how do I correct the query to get the desired result? It is desirable that I do not pull back all the data from the database.

Answer 1

I think you may need to add one more select to your query:

var query = Items.GroupBy(review => review.Product)
.Select(group => new
{
    Product = group.Key,
    Rating = group.Average(review => review.Rating)
})
.OrderByDescending(pr => pr.Rating)
.Select(x => new { Product = x.Product, Rating = x.Rating })
.Skip(itemsToSkip)
.Take(count)
.ToList();