如何在“WHERE”条件下使用List编写LINQ to Entities查询

Question

I would like to know to to write most efficient LINQ ( EDIT : to Entities) query with a list as a condition. Here is the thing.

Lets say we have the following data structure:

 public class Recipe
 {
   public int Id;
   public string Name;
   public List<Ingredient> IngredientList;
 }

 public class Ingredient
 {
   public int Id;
   public string Name;
 }

Now, I would like to make a query which will search all the Recipes which have ALL given ingredients.

 public List<Recipe> GetRecipesWhichHaveGivenIngredients(List<Ingredients> ingredients)
 {
   List<Recipe> recipes;

   using (DataContext context = new DataContext())
   {
    //efficient LINQ query goes here
    recipes = context.Recipes.Where(recipe => /*medaCode recipe.IngredientList.Contains(ingredients) */).ToList();
   }
   return recipes;
 }

Basically this is the problem how to determine whether a given set is a subset of another set.

I have tried with the following query (the main idea is usage of the Intersect operation):

List<Recipe> recipes = dataContext.Recipes.Include("Ingrediens").Where(rec => rec.IngredientList.Select(ingr => ingr.Id).Intersect(ingredients.Select(sy =>  sy.Id)).Count() == ingredients.Count).ToList();

But I get the following error:

Unable to create a constant value of type 'Closure type'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

Answer 1

well, if IngredientList really is a List<T> , you'll be able to do:

recipes = context.Recipes.Where(recipe => recipe.IngredientList.Exists(i => i.Id == ingredient.Id)).ToList();

but that means all the list needs to be populated. Since this looks like a LINQ to SQL query, I'm guessing IngredientList is just a connected table...? In that case, you won't have the full list, but you'll still be able to do something similar:

recipes = context.Recipes.Where(recipe => recipe.IngredientList.Count(i => i.Id == ingredient.Id) > 0).ToList();

...and it should still just query the sql server once.

EDIT

As was just pointed out in comments, this doesn't quite answer the question. As for contains-all search, I don't think it can be done without looping through the input. The good thing is that this can be done without enumerating the IEnumerable recipes , so the below code will still just hit the sql server once, with a single query:

var recipes = context.Recipes.AsEnumerable<Recipe>();

ingredients.ForEach(i =>
    var recipes = recipes.Where(r =>
        r.IngredientList.Count(ii => ii.Id == i.Id) > 0
    );
);

return recipes.ToList();

The query will not be executed until ToList() is hit.

Answer 2

Don't use a List<Ingredient> for the ingredients that you want to find; use a HashSet<Ingredient> and the IsProperSubsetOf method, which accepts a collection as its argument:

.Where(x => ingredients.IsProperSubsetOf(x.IngredientList))

In addition to being an O(n+m) operation, this has the added benefit of being code that tells you what it's doing when you look at it.

Edit

In case the above is not clear:

public List<Recipe> GetRecipesWhichHaveGivenIngredients(HashSet<Ingredient> ingredients)
{
   using (DataContext context = new DataContext())
   {
       return context.Recipes
           .Where(x => ingredients.IsProperSubsetOf(x.IngredientList)  
           .ToList();
   }
 }

Answer 3

don't know if this will work in Linq2SQL, but in Linq2Object, this works:

public static class Util
{
    public static List<Recipe> GetRecipesWhichHaveGivenIngredients(this List<Recipe> recipies, List<Ingredient> ingredients)
    {
        int icount=ingredients.Count;

        var res = recipies.Where(r => r.IngredientList.Where(i => ingredients.Contains(i)).Count() == icount).ToList();
        return res;
    }
}