为 Entity Framework 多对多关系构建表达式树？

Question

我正在为以下结构编写代码：
3 桌

Table "Book", "BookAuthor" and "Author".

我现在想构建一个 ExpressionTree 以获得以下 Linq 结果：

_ent.Book.Where(c => c.BookAuthor.Any(cd => cd.Author.AuthorName == "test"));

我的问题是到达 cd.Author.AuthorName（或类似）的最后一个表达式。

我当前的代码：

private MethodCallExpression BuiltMethodCall(IQueryable _query, string CustTypeID, Type _ParentObjType,
Type _ChildObjType, string strChildObj, string strChildCol)
    {

        //This function will build a dynamic linq expression tree representing the ling calls of:
        //Book.Where(c => c.BookAuthor.Any(cd => cd.Author = custTypeID))

        ConstantExpression value = Expression.Constant(CustTypeID);

        //Build the outer part of the Where clause
        ParameterExpression parameterOuter = Expression.Parameter(_ParentObjType, "c");
        MemberExpression propertyOuter = Expression.Property(parameterOuter, strChildObj);

        //Build the comparison inside of the Any clause
        ParameterExpression parameterInner = Expression.Parameter(_ChildObjType, "cd");
        MemberExpression propertyInner = Expression.Property(parameterInner, strChildCol);

        BinaryExpression comparison = Expression.Equal(propertyInner, value);
        LambdaExpression lambdaInner = Expression.Lambda(comparison, parameterInner);

        //Create Generic Any Method
        Func<MethodInfo, bool> methodLambda = m => m.Name == "Any" && m.GetParameters().Length == 2;
        MethodInfo method = typeof(Enumerable).GetMethods().Where(methodLambda).Single()
            .MakeGenericMethod(_ChildObjType);

        //Create the Any Expression Tree and convert it to a Lambda
        MethodCallExpression callAny = Expression.Call(method, propertyOuter, lambdaInner);
        LambdaExpression lambdaAny = Expression.Lambda(callAny, parameterOuter);

        //Build the final Where Call
        MethodCallExpression whereCall = Expression.Call(typeof(Queryable), "Where",
            new Type[] { _query.ElementType }, new Expression[]
            {
                    _query.Expression,
                    Expression.Quote(lambdaAny)
            });

        return whereCall;
    }

Answer 1

如果我没理解错的话，你问的是如何创建嵌套的属性访问器表达式。

借助 String.Split and Enumerable.Aggregate 方法非常简单。

这是一个自定义扩展方法，可以用作 Expression.Property 方法替换：

public static partial class ExpressionExtensions
{
    public static MemberExpression Property(this Expression instance, string path)
    {
        return (MemberExpression)path.Split('.').Aggregate(instance, Expression.Property);
    }
}

它可以正确处理在 string 中使用 . 分隔符（例如 "BookAuthor" 或 "Author.AuthorName"）编码的简单属性和嵌套属性。

所以不用

MemberExpression propertyOuter = Expression.Property(parameterOuter, strChildObj);

和

MemberExpression propertyInner = Expression.Property(parameterInner, strChildCol);

你可以放心使用

MemberExpression propertyOuter = parameterOuter.Property(strChildObj);

和

MemberExpression propertyInner = parameterInner.Property(strChildCol);

为 Entity Framework 多对多关系构建表达式树？

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