EF Core 全局查询过滤器复杂表达式答案

【问题标题】：EF Core Global Query Filter Complex expressionEF Core 全局查询过滤器复杂表达式
【发布时间】：2021-12-25 03:57:19
【问题描述】：

我正在尝试在我的应用程序中实现一个用于租赁的全局查询过滤器。我有AssessmentModels，它可以有多个所有者，这些所有者来自我无法完全控制的第三方，但可以根据需要进行调整。（并且可以在我保存到我的数据库之前进行操作）现在，Owners 存储为分号分隔的字符串（例如，team1;team2）。

我提出了以下适用于选择数据但不适用于全局查询过滤器的方法：

private Expression<Func<AssessmentModel, bool>> GetAssessmentFilter()
{
    // The lambda parameter.
    var assessmentParameter = Expression.Parameter(typeof(AssessmentModel), "a");
    // Build the individual conditions to check against.
    var orConditions = _adminTeamNames
        .Select(keyword => (Expression<Func<AssessmentModel, bool>>)(a => EF.Functions.Like(a.Owners, $"%{keyword}%")))
        .Select(lambda => (Expression)Expression.Invoke(lambda, assessmentParameter))
        .ToList();

    // Combine the individual conditions to an expression tree of nested ORs.
    var orExpressionTree = orConditions
        .Skip(1)
        .Aggregate(
            orConditions.First(),
            (current, expression) => Expression.OrElse(expression, current));


    // Build the final predicate (a lambda expression), so we can use it inside of `.Where()`.
    var predicateExpression = (Expression<Func<AssessmentModel, bool>>)Expression.Lambda(
        orExpressionTree,
        assessmentParameter);

    return predicateExpression;
}

所以， var result = db.Assessments.Where(predicateExpression).ToList(); 有效，但 modelBuilder.Entity<AssessmentModel>().HasQueryFilter(predicateExpression); 给出错误：

System.InvalidOperationException : The LINQ expression 'DbSet<AssessmentModel>()
.Where(a => Invoke(a => value(DbFunctions).Like(a.Owners, "%Los%"), a)
 || Invoke(a => value(DbFunctions).Like(a.Owners, "%Atl%"), a)
)' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

【问题讨论】：

标签： c# entity-framework-core .net-6.0 ef-core-6.0

【解决方案1】：

AFAIK EF Core 不擅长翻译调用表达式。尝试用您的自定义参数表达式（即assessmentParameter）手动替换构建 lambda 的参数：

var orConditions = _adminTeamNames
    .Select(keyword => (Expression<Func<AssessmentModel, bool>>)(a => EF.Functions.Like(a.Owners, $"%{keyword}%")))
    .Select(lambda => new ReplacingExpressionVisitor(lambda.Parameters, new []{assessmentParameter}).Visit(lambda.Body))
    .ToList();

【讨论】：