使用 String.Contains 的 EF Core 查询

Question

我们需要在逗号分隔的字符串中搜索给定的术语。构建查询以便它忽略逗号分隔字符串中可能的前导和尾随空格。 我想出了以下查询，它在 EF 6.0 上运行良好

var trimmedTags = tags.Select(t => t.Trim()); // List of tags we need to look for    
return products.Where(p => trimmedTags.Any(t => ("," + p.Categories + ",").Contains("," + t + ",")) ||
                               trimmedTags.Any(t => ("," + p.Categories + ",").Contains(", " + t + ",")) ||
                               trimmedTags.Any(t => ("," + p.Categories + ",").Contains("," + t + " ,")) ||
                               trimmedTags.Any(t => ("," + p.Categories + ",").Contains(", " + t + " ,")));

此查询不再在 EF Core 3.1 中运行并引发以下错误：

System.InvalidOperationException: 'The LINQ expression 'DbSet<Product>
    .Where(p => __trimmedTags_1
        .Any(t => ("," + p.Categories + ",").Contains("," + t + ",")) || __trimmedTags_1
        .Any(t => ("," + p.Categories + ",").Contains(", " + t + ",")) || __trimmedTags_1
        .Any(t => ("," + p.Categories + ",").Contains("," + t + " ,")) || __trimmedTags_1
        .Any(t => ("," + p.Categories + ",").Contains(", " + t + " ,")))' 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(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.'

我的目标表有数百万行，因此很遗憾无法进行客户评估。 EF Core 团队声称支持string.Contains，但我不明白为什么我的查询在 EF Core 中突然失败。

Answer 1

这个问题的不同变体经常出现在 SO 上，而且问题总是相同的 - 即使在最新版本 (5.x) 中，EF Core 也不支持内存中集合上的运算符，而不是简单的 @ 987654322@ 具有原始值（或Any 可以转换为Contains，如x => memValues.Any(v => v == SomeExpr(x))，== 运算符是必不可少的）。

解决方法也是一样的——动态构建表达式——||（或）基于Any和&&（和）基于All。

这种情况需要||，并且类似于How to simplify repetitive OR condition in Where(e => e.prop1.contains() || e.prop2.contains() || ...)，但交换了值和字段角色，所以以下是我将使用的辅助方法：

public static partial class QueryableExtensions
{
    public static IQueryable<T> WhereAnyMatch<T, V>(this IQueryable<T> source, IEnumerable<V> values, Expression<Func<T, V, bool>> match)
    {
        var parameter = match.Parameters[0];
        var body = values
            // the easiest way to let EF Core use parameter in the SQL query rather than literal value
            .Select(value => ((Expression<Func<V>>)(() => value)).Body)
            .Select(value => Expression.Invoke(match, parameter, value))
            .Aggregate<Expression>(Expression.OrElse);
        var predicate = Expression.Lambda<Func<T, bool>>(body, parameter);
        return source.Where(predicate);
    }
}

请注意，这仅适用于顶级查询表达式。如果您需要这样的东西作为查询表达式树的一部分（如集合导航属性），您需要不同类型的辅助函数或允许表达式注入的库。

幸运的是，这里不是这种情况，因此可以通过传递trimmedTags 和每个标签值的条件来直接使用上述辅助方法，例如

return products.WhereAnyMatch(trimmedTags, (p, t) => 
    ("," + p.Categories + ",").Contains("," + t + ",") ||
    ("," + p.Categories + ",").Contains(", " + t + ",") ||
    ("," + p.Categories + ",").Contains("," + t + " ,") ||
    ("," + p.Categories + ",").Contains(", " + t + " ,"));