linq - PredicateBuilder可以生成跨越多个表的谓词吗？

我想动态生成一个谓词，这些谓词跨越Linq语句中的Join跨越多个表。在以下代码片段中，我想使用PredicateBuilder或类似的结构来替换以下代码中的“where”语句:

代替:

public class Foo
{
    public int FooId;  // PK
    public string Name;
}

public class Bar
{
    public int BarId;  // PK
    public string Description;
    public int FooId;  // FK to Foo.PK
}

void Test()
{
    IQueryable<Foo> fooQuery = null;    // Stubbed out
    IQueryable<Bar> barQuery = null;    // Stubbed out

    IQueryable<Foo> query =
        from foo in fooQuery
        join bar in barQuery on foo.FooId equals bar.FooId
        where ((bar.Description == "barstring") || (foo.Name == "fooname"))
        select foo;
}

用类似的东西:

void Test(bool searchName, bool searchDescription)
{
    IQueryable<Foo> fooQuery = null;    // Stubbed out
    IQueryable<Bar> barQuery = null;    // Stubbed out

    IQueryable<Foo> query =
        from foo in fooQuery
        join bar in barQuery on foo.FooId equals bar.FooId
        select foo;

    // OR THIS

    var query =
        from foo in fooQuery
        join bar in barQuery on foo.FooId equals bar.FooId
        select new {foo, bar};

    var predicate = PredicateBuilder.False<Foo>();
    if (searchName)
    {
        predicate = predicate.Or(foo => foo.Name == "fooname");
    }
    if (searchDescription)
    {
        // Cannot compile
        predicate = predicate.Or(bar => bar.Description == "barstring");
    }
    // Cannot compile
    query = query.Where(predicate);
}

解决这个问题有什么想法，想法和策略吗？

谢谢，

欧拉运算符(operator)

最佳答案

我认为您的问题出在PredicateBuilder的类型T上-谓词的一半(如果对Foo进行操作)，另一半则在Bar上。

您可以将其替换为简单的手动构建的查询:

void Test()
{
    IQueryable<Foo> fooQuery = null;    // Stubbed out
    IQueryable<Bar> barQuery = null;    // Stubbed out

    IQueryable<Foo> query =
        from foo in fooQuery
        join bar in barQuery on foo.FooId equals bar.FooId
        select new {Foo = foo, Bar = bar};

    if (searchName)
    {
        query = query.Where(fb => fb.Foo.Name == "fooname");
    }
    if (searchDescription)
    {
        query = query.Where(fb => fb.Bar.Description == "barstring");
    }

    // use query here
}

另一种方法是使用PredicateBuilder，但使其在Foo，Bar对上起作用-例如

class FooBar
{
   public Foo Foo {get;set;}
   public Bar Bar {get;set;}
}

void Test(bool searchName, bool searchDescription)
{
    IQueryable<Foo> fooQuery = null;    // Stubbed out
    IQueryable<Bar> barQuery = null;    // Stubbed out

    var query =
        from foo in fooQuery
        join bar in barQuery on foo.FooId equals bar.FooId
        select new FooBar
        {
           Foo = foo,
           Bar = bar
        };

    var predicate = PredicateBuilder.False<FooBar>();
    if (searchName)
    {
        predicate = predicate.Or(foobar => foobar.Foo.Name == "fooname");
    }
    if (searchDescription)
    {
        predicate = predicate.Or(foobar => foobar.Bar.Description == "barstring");
    }
    query = query.Where(predicate);

    // use query here
}

关于linq - PredicateBuilder可以生成跨越多个表的谓词吗？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/5303269/