实体框架 | 包括 -> 选择空字段

Question

I have a problem with our Select.

But first, we have 3 tables: for example:

inventory, items, device_properties

The inventory have Items. And a few of the Items have device_properties.

The items and the device_properties doesnt change. In this case we include fix the device_properties to the items.

Now at the call we want to get the inventory, and include at the request an include from items to the inventory entries.

Now the problem is, if we request it, we send data like x.items.device_properties.name But this can be null. And if this is null it doesnt work because:

nullable object must have a value

and if i do like the following, he needs to long:

deviceName = x.items.device_properties == null ? "" : x.items.device_properties.name

var items = db.inventory.Where(x => x.itemName== itemName).Include(i => i.items).Select(x => new
{
  itemId = x.id,
  x.itemName,
  deviceName = x.items.device_properties.name
}).AsNoTracking().ToList();

At this i get the message, that the nullable needs a value. And if i do it like:

var items = db.inventory.Where(x => x.itemName== itemName).Include(i => i.items).Select(x => new
{
  itemId = x.id,
  x.itemName,
  deviceName = x.items.device_properties == null ? "" : x.items.device_properties.name
}).AsNoTracking().ToList();

The query needs a lot longer

message

System.InvalidOperationException: Nullable object must have a value

Answer 1

In addition device_properties should not be null , you should check items property should not be null .

var items = db.inventory.Where(x => x.itemName== itemName).Include(i => i.items).Select(x => new
{
  itemId = x.id,
  x.itemName,
  deviceName = x.items == null || x.items.device_properties == null ? "" : x.items.device_properties.name
}).AsNoTracking().ToList();