Does anybody know how to apply a "where in values" type condition using LINQ-to-Entities? I've tried the following but it doesn't work:
var values = new[] { "String1", "String2" }; // some string values
var foo = model.entitySet.Where(e => values.Contains(e.Name));
I believe this works in LINQ-to-SQL though? Any thoughts?
It is somewhat of a shame that Contains is not supported in Linq to Entities.
IN and JOIN are not the same operator (Filtering by IN never changes the cardinality of the query).
Update: found out how to do this. And EF will generate the appropriate SQL on the database. I'm not sure if this is for EF4 only but I got the tip from Entity Framework 4.0 Recipes
var listOfIds=GetAListOfIds();
var context=CreateEntityFrameworkObjectContext();
var results = from item in context.Items
where listOfIds.Contains(item.Category.Id)
select item;
//results contains the items with matching category Ids
This query generates the correct in clause on the server side. I haven't tested it with EF 3.5 but it does work with EF4.
NB: The values passed into the in clause are NOT parameters so make sure you validate your inputs.
It is somewhat of a shame that Contains is not supported in Linq to Entities.
IN and JOIN are not the same operator (Filtering by IN never changes the cardinality of the query).
FYI:
If you are using ESql you are able to use in operation. I don't have VS 2008 With me but code should be something like following:
var ids = "12, 34, 35";
using (context = new Entites())
{
var selectedProducts = context.CreateQuery<Products>(
String.Format("select value p from [Entities].Products as p
where p.productId in {{{0}}}", ids)).ToList();
...
}
For the cases when you want to use expressions when querying your data, you can use the following extension method (adapted after http://social.msdn.microsoft.com/forums/en-US/adodotnetentityframework/thread/095745fe-dcf0-4142-b684-b7e4a1ab59f0/):
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Data.Objects;
namespace Sample {
public static class Extensions {
public static IQueryable<T> ExtWhereIn<T, TValue>(this ObjectQuery<T> query,
Expression<Func<T, TValue>> valueSelector,
IEnumerable<TValue> values) {
return query.Where(BuildContainsExpression<T, TValue>(valueSelector, values));
}
public static IQueryable<T> ExtWhereIn<T, TValue>(this IQueryable<T> query,
Expression<Func<T, TValue>> valueSelector,
IEnumerable<TValue> values) {
return query.Where(BuildContainsExpression<T, TValue>(valueSelector, values));
}
private static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(
Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values) {
if (null == valueSelector) { throw new ArgumentNullException("valueSelector"); }
if (null == values) { throw new ArgumentNullException("values"); }
ParameterExpression p = valueSelector.Parameters.Single();
// p => valueSelector(p) == values[0] || valueSelector(p) == ...
if (!values.Any()) {
return e => false;
}
var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));
var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));
return Expression.Lambda<Func<TElement, bool>>(body, p);
}
}
class Program {
static void Main(string[] args) {
List<int> fullList = new List<int>();
for (int i = 0; i < 20; i++) {
fullList.Add(i);
}
List<int> filter = new List<int>();
filter.Add(2);
filter.Add(5);
filter.Add(10);
List<int> results = fullList.AsQueryable().ExtWhereIn<int, int>(item => item, filter).ToList();
foreach (int result in results) {
Console.WriteLine(result);
}
}
}
}
Using the extensions is really easy (as you can see in the sample). To use it on a database object, assuming you are filtering a table called "Product" by more than one id, you could do something like that:
class Product {
public int Id { get; set; }
/// ... other properties
}
List<Product> GetProducts(List<int> productIds) {
using (MyEntities context = new MyEntities()) {
return context.Products.ExtWhereIn<Product, int>(product => product.Id, productIds).ToList();
}
}
Yes it does translate to SQL, it generates a standard IN statement like this:
SELECT [t0].[col1]
FROM [table] [t0]
WHERE [col1] IN ( 'Value 1', 'Value 2')
Using the where method doesn't alway work
var results = from p in db.Products
where p.Name == nameTextBox.Text
select p;
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