使用EF Core将SQL转换为Linq

Question

I am using .NET Core 2.2, EF Core, C# and SQL Server 2017. I am not able to translate the query I need to Linq.

This is the query I need to convert:

SELECT      TOP 5
            p.Id, 
            p.Title, 
            AVG(q.RatingValue) AvgRating
FROM        Movies AS p
INNER JOIN  Ratings AS q ON p.Id = q.MovieId
GROUP BY    p.Id, p.Title
ORDER BY    AvgRating DESC, p.Title ASC

The idea of the previous query is to get the Top 5 movies according to the Avg rating, ordering it by the highest average first, and in case of same average order alphabetically.

So far this is my query that makes the join, but then still missing: the group by, average, and ordering:

public class MovieRepository : IMovieRepository
{
    private readonly MovieDbContext _moviesDbContext;
    public MovieRepository(MovieDbContext moviesDbContext)
    {
        _moviesDbContext = moviesDbContext;
    }

    public IEnumerable<Movie> GetTopFive()
    {
        var result = _moviesDbContext.Movies.OrderByDescending(x => x.Id).Take(5).
                     Include(x => x.Ratings);

        return result;
    }
}

And these are the entities:

 public class Movie
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int YearOfRelease { get; set; }
    public string Genre { get; set; }
    public int RunningTime { get; set; }
    public IList<Rating> Ratings { get; set; }
}

public class Rating
{
    public int Id { get; set; }
    public int MovieId { get; set; }
    public int UserId { get; set; }
    public decimal RatingValue { get; set; }
}

I tried to use Linqer tool also to convert my query to Linq, but it was not working.

I will appreciate any help to convert that query to LINQ for the method "GetTopFive".

Thanks

Answer 1

Try this one -

var data = _moviesDbContext.Movies.Include(x => x.Ratings)
            .Select(x => new {
                Id = x.Id,
                Title = x.Title,
                Average = (int?)x.Ratings.Average(y => y.RatingValue)
        }).OrderByDescending(x => x.Average).ThenBy(x => x.Title).Take(5).ToList();

Answer 2

Try as follows:

public IEnumerable<Movie> GetTopFive()
{
    var result = _moviesDbContext.Ratings.GroupBy(r => r.MovieId).Select(group => new
        {
            MovieId = group.Key,
            MovieTitle = group.Select(g => g.Movie.Title).FirstOrDefault(),
            AvgRating = group.Average(g => g.RatingValue)
        }).OrderByDescending(s => s.AvgRating).Take(5).ToList();
    return result;
}

This will exclude the movies having no ratings.

But if you do as follows (as artista_14's answer):

public IEnumerable<Movie> GetTopFive()
{
    var result = _moviesDbContext.Movies.GroupBy(x => new { x.Id, x.Title })
        .Select(x => new {
            Id = x.Key.Id,
            Title = x.Key.Title,
            Average = x.Average(y => y.Ratings.Sum(z => z.RatingValue))
    }).OrderByDescending(x => x.Average).ThenBy(x => x.Title).Take(5).ToList();
    return result;
}

this will include the movies having no ratings also.

Note: I see your Rating model class does not contain any Movie navigation property. Please add this as follows:

public class Rating
{
    public int Id { get; set; }
    public int MovieId { get; set; }
    public int UserId { get; set; }
    public decimal RatingValue { get; set; }

    public Movie Movie { get; set; }
}

Answer 3

and finally this is the code working nicely:

var data = _moviesDbContext.Movies.Include(x => x.Ratings)
            .Select(x => new MovieRating
            {
                Id = x.Id,
                Title = x.Title,
                Average = x.Ratings.Average(y => y.RatingValue)
            }).OrderByDescending(x => x.Average).ThenBy(x => x.Title).Take(5).ToList();

        return data;

The problem was creating an anonymous type in the select, so this line resolves the issue: .Select(x => new MovieRating

And this is the complete code for the method and the new class I have created to map the select fields with a concrete type:

public class MovieRepository : IMovieRepository
{
    private readonly MovieDbContext _moviesDbContext;
    public MovieRepository(MovieDbContext moviesDbContext)
    {
        _moviesDbContext = moviesDbContext;
    }

    public IEnumerable<Movie> GetAll()
    {
        return _moviesDbContext.Movies;
    }

    public IEnumerable<MovieRating> GetTopFive()
    {
        var result = _moviesDbContext.Movies.Include(x => x.Ratings)
                    .Select(x => new MovieRating
                    {
                        Id = x.Id,
                        Title = x.Title,
                        Average = x.Ratings.Average(y => y.RatingValue)
                    }).OrderByDescending(x => x.Average).ThenBy(x => x.Title).Take(5).ToList();

        return result;
    }
}

public class MovieRating
{
    public int Id { get; set; }
    public string Title { get; set; }
    public decimal Average { get; set; }
}