using linq to find if a text field contains any string in a list

问题

im running this in asp.net core v3.1

my question is similar to this question:
How to use Linq to check if a list of strings contains any string in a list

with the specific question relating to the first answer such that

filterTags = ["abc", "cd", "efg"]
var results = db.People
                .Where(p => filterTags.Any(tag => p.Tags.Contains(tag)));

so basically saying
give me results from the db of all People
who's Tags field contains any of the filterTags
where Tags = a big text field populated by a bunch of space-delimited tags

This seems straightforward (esp since this has been written before)
but i get an error back

System.InvalidOperationException: The LINQ expression 'DbSet .Where(p => __filterTags_0 .Any(tag => p.Tags.Contains(tag)))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()

does anyone know what this means or what im doing wrong?

回答1:

This is not possible with pure EF LINQ. You have to create helper which transforms your search list in Expression Tree.

public static class QueryExtensions
{
    private static MethodInfo _containsMethodInfo = typeof(string).GetMethod("Contains")!;

    public static IQueryable<T> FilterUsingContains<T>(this IQueryable<T> query, Expression<Func<T, string>> prop, IList<string> items)
    {
        if (items.Count == 0)
            return query.Where(e => 1 == 2);

        var param = prop.Parameters[0];

        var predicate = items.Select(i =>
                (Expression)Expression.Call(prop.Body, _containsMethodInfo, Expression.Constant(i, typeof(string))))
            .Aggregate(Expression.OrElse);

        var lambda = Expression.Lambda<Func<T, bool>>(predicate, param);

        return query.Where(lambda);
    }
}

Then you can use this extension in your queries

filterTags = ["abc", "cd", "efg"]
var results = db.People
    .Where(p => p.Tags.AsQueryable().FilterUsingContains(t => t, filterTags).Any());

回答2:

Here is a workaround for you:

using System.Linq; 

string[] filterTags = {"abc", "cd", "efg"};
var results = db.People.Where(p => filterTags.Contains(p.Tags)).ToList();

来源：https://stackoverflow.com/questions/65193835/using-linq-to-find-if-a-text-field-contains-any-string-in-a-list

标签

linq

asp.net-core

contains

any