从Entity Framework中的不同表格获取平均值

Question

I currently have several tables, but the relevant ones are Restaurant and Review . Each restaurant can have multiple reviews, but a review has only 1 restaurant.

Now when I retrieve restaurants from the database, I want their average rating from the reviews on each restaurant.

I select the restaurants based on distance to a given location.

Currently, I have gotten this far:

public IEnumerable<Restaurant> GetRestaurantsCloseToCoords(DbGeography coordinates, int amountOfRestaurants)
{
    using (var ctx = _context)
    {
        var data = ctx.Restaurants.OrderBy(x => x.Address.Coordinates.Distance(coordinates))
            .Take(amountOfRestaurants)
            .ToList();

        return data;
    }
}

My restaurant object looks like this:

public class Restaurant
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public Address Address { get; set; }
    public List<Tag> Tags { get; set; } = new List<Tag>();
    public int PriceRange { get; set; }
    public double AverageRating { get; set; }
}

I can use a for loop and calculate an average for each restaurant in that list, but I'm pretty sure it's very slow.

Is there anyone who could help me? I'd prefer to keep this in LINQ, but other techinques are totally fine, too!

Let me know if I should elaborate more, thanks in advance!

Answer 1

With huge number of Reviews , no matter what optimized code you write, things will slow down eventually.

A better approach would be to add a new column to the Restaurant table, called AverageRating . Then write a piece of code/script (a windows service or sql job) that runs daily (or periodically) and updates the values in this column. (You may optimize this based on various factors like, which hotel got new ratings on that day, etc)

This way:

1. You would have averages ready to merely filter using a Linq.
2. You would not calculate all the averages again, simply because there was another call to your method.
3. A significant amount resources would be thus saved.

Yes, this would be time consuming to implement, but yeah, would solve the issue at hand in long term

Answer 2

using (var ctx = _context)
{
    var data = ctx.Restaurants
            .OrderBy(x => x.Address.Coordinates.Distance(coordinates))
            .Take(amountOfRestaurants)
            .Select(t=> new { Restaurant = t, Rating = ctx.Reviews.Where(c=>c.RestaurantId == t.Id).Select(c=>c.Rating).Avg()})
            .ToList();

    foreach(t in data)
    {
        t.Restaurant.AverageRating = t.Rating;
    }

    return data.Select(t=>t.Restaurant);
}