遍历IQueryable而不调用ToList（）

Question

I have a DB used for a production line. It has an Orders table, and Ordertracker table, an Item table, and an Itemtracker table.

Both Orders and Items have many-to-many relationships with status. The tracker tables resolves these relationships in such a way that an item can have multiple entries in the tracker - each with a particular status.

I tried to upload a picture of the tables to make things clearer but alas, I don't have enough points yet :C

I need to find items whose last status in the Itemtracker table meets a condition, either '3' or '0'.

I then need to get the first one of these items.

The steps I am using to accomplish this are as follows:

1. Get all the Orders which have a certain status.
2. Get all the Items in that Order.
3. Get all the Items whose last status was = 0 or 3.
4. Get the first of these items.

My code is as follows:

    public ITEM GetFirstItemFailedOrNotInProductionFromCurrentOrder()
    {

        var firstOrder = GetFirstOrderInProductionAndNotCompleted();

        var items = ERPContext.ITEM.Where(i => i.OrderID == firstOrder.OrderID) as IQueryable<ITEM>;

        if (CheckStatusOfItems(items) != null)
        {
            var nextItem = CheckStatusOfItems(items);

            return nextItem ;
        }
        else
        {
            return null;
        }
    }

    private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
    {
        List<ITEM> listOfItemsToProduce = new List<ITEM>();

        foreach (ITEM item in items.ToList())
        {

           var lastStatusOfItem = ERPContext.ITEMTRACKER.Where(it => it.ItemID == item.ItemID)
                                                        .OrderByDescending(it => it.ItemTrackerID).FirstOrDefault();

            if (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
            {
                listOfItemsToProduce.Add(item);
            }
        }
        return listOfItemsToProduce.FirstOrDefault();

    }

Now, this all works fine and returns what I need but I'm aware that this might not be the best approach. As it is now my IQueryable collection of items will never contain more than 6 items - but if it could grow larger, then calling ToList() on the IQueryable and iterating over the results in-memory would probably not be a good idea.

Is there a better way to iterate through the IQueryable items to fetch out the items that have a certain status as their latest status without calling ToList() and foreaching through the results?

Any advice would be much appreciated.

Answer 1

Using LINQ query syntax, you can build declaratively a single query pretty much the same way you wrote the imperative iteration. foreach translates to from , var to let and if to where :

private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
{
    var query =
        from item in items
        let lastStatusOfItem = ERPContext.ITEMTRACKER
            .Where(it => it.ItemID == item.ItemID)
            .OrderByDescending(it => it.ItemTrackerID)
            .FirstOrDefault()
        where (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
        select item;

    return query.FirstOrDefault();
}

or alternatively using from instead of let and Take(1) instead of FirstOrDefault() :

private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
{
    var query =
        from item in items
        from lastStatusOfItem in ERPContext.ITEMTRACKER
            .Where(it => it.ItemID == item.ItemID)
            .OrderByDescending(it => it.ItemTrackerID)
            .Take(1)
        where (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
        select item;

    return query.FirstOrDefault();
}