LINQ to SQL复杂查询

Question

I have the following object

public Cell {
    public string CategoryId {get; set;}
    public DateTime FromTime {get; set}
    public DateTime ToTime {get; set}
}

I have a DB table that is called Item that looks like this:

Item

-------------------------
CategoryId    LastUpdated

Now in the code I have a list of Cell List<Cell> ToBeFetchedFromDB that contains more than one Cell, suppose the list contains Foo and Bar , I want to dynamically build a query like this BY INTERATING THROUGH THE COLLECTION ToBeFetchedFromDB WITHIN MY LINQ TO SQL QUERY instead of statically constructing the query:

from x in Item
where x.CategoryId == Foo.CategoryId && Foo.FromTime < x.LastUpdated < Foo.ToTime
      || x.CategoryId == Bar.CategoryId && Bar.FromTime < x.LastUpdated < Bar.ToTime
select x

I have been trying but can't figure it out :(

Thanks guys!

Answer 1

Using PredicateBuilder you can go through each item in your list using a foreach and add a new option for your filter:

var filter = PredicateBuilder.False<Item>();

foreach (var cell in ToBeFetchedFromDB)
{
    filter = PredicateBuilder.Or(filter, item => 
        item.CategoryId == cell.CategoryId &&
        cell.FromTime < item.LastUpdated &&
        item.LastUpdated < cell.ToTime);
}

var query = Item.Where(filter);

A copy of PredicateBuilder from the link:

public static class PredicateBuilder
{
    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
                                                        Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
                (Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
                                                            Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
                (Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
    }
}