c# - 实体子项上 where 条件的 Linq 表达式

Question

你好，

如何通过 linq 表达式构建以下语句以便将其转换为 sql？

builder.Where(e => e.SomeChild.Name.ToLower() == "something");

builder 是一个 DbContext。当我直接调用它时，它被正确解释为这样

select ... from someTable 
inner join childTable on ... 
where LOWER(childTable.Name) = @someParam

所以我以这种方式构建 where 条件:

private readonly Expression<Func<TEntity, TProperty>> _property;

private Expression<Func<TEntity, bool>> insensitiveEqualityPredicate(string formula)
        {
            var parameter = Expression.Parameter(typeof(TEntity));
            var property = (PropertyInfo)((MemberExpression)_property.Body).Member;
            var propertyParameter = Expression.Parameter(property.DeclaringType);
            var lowerExpression = Expression.Call(
                    Expression.Invoke(_property, propertyParameter),
                    typeof(string).GetMethods().Where(m => m.Name == "ToLower" && m.GetParameters().ToList().Count == 0).FirstOrDefault()
                    );
            return Expression.Lambda<Func<TEntity, bool>>(
                Expression.Equal(
                    lowerExpression,
                    Expression.Constant(formula.ToLower(), typeof(string))
                ),
                parameter
            );
        }

---

它就像魅力一样:

_property = e => e.Name;
...
builder.Where(insensitiveEqualityPredicate("whatever"));

但不适用于 child 条件:

_property = e => e.SomeChild.Name;
...
builder.Where(insensitiveEqualityPredicate("whatever"));

因为:

The LINQ expression 'Param_0.Name.Equal("whatever")' could not be translated and will be evaluated locally.

什么是正确的做法？

提前谢谢你。

Answer 1

据我了解，您有一个字段 _property类型 Expression<Func<TEntity, TProperty>> (在这种特殊情况下，它实际上是 Expression<Func<TEntity, string>> )，它是用类似 e => e.Name 的 lambda 表达式初始化的, e => e.SomeChild.Name等等

为了构建所需的谓词，您可以重用 _property表达式参数并使用 Body 属性作为新表达式的基础:

private readonly Expression<Func<TEntity, TProperty>> _property;

private Expression<Func<TEntity, bool>> insensitiveEqualityPredicate(string formula)
{
    return Expression.Lambda<Func<TEntity, bool>>(
        Expression.Equal(
            Expression.Call(_property.Body, "ToLower", Type.EmptyTypes),
            Expression.Constant(formula.ToLower(), typeof(string))
        ),
        _property.Parameters
    );
}