急切加载的实体框架问题

Question

I want to create a new TrackingItem , but response from the server is:

I don't know how to include the Tenant property.

Answer 1

When working with navigation properties, populate the navigation properties, not just the FKs.

The issue you are likely seeing is that when you call code to add a Tracking Item, the DbContext will start tracking that instance, however you have only set the TenantId on that instance, not the Tenant Navigation property. Any code that gets that TrackingItem reference from the DbContext and tries to go to.Tenant.* will get that exception because while a TenantID was provided, no Tenant was loaded. For a new instance I don't believe lazy loading would even kick in, and you don't appear to be enabling lazy loading to begin with. (Tenant is not declared as virtual )

Now doing this with generic repositories is a pain since you've wrapped the DbContext in a Repository that's specific to Tracking Items, so that will need some sorting out. With a DbContext reference it would need to look something like:

var tenant = _context.Tenants.Single(x => x.TenantId == tenantId);
var trackingItem = new TrackingItem
{
    Tenant = tenant,
    // .. remaining TrackingItem fields...
}
_context.TrackingItems.Add(trackingItems);
await _context.SaveChangesAsync();

When using navigation properties, I recommend not declaring FK properties, but instead using shadow properties for the FK. The issue with declaring FKs alongside navigation properties is that you have 2 sources of truth for the relationship (.TenantId and.Tenant.TenantId) which can be changed independently of one another. The behaviour when editing the FK can also vary depending on whether the navigation property is loaded or not.

It also means whenever you fetch a TrackingInstance, if the code is going to want to check the Tenant either lazy loading needs to be initialized or you need to remember to eager load Tenant when loading the tracking instance:

var trackingItem = _context.TrackingItems
    .Include(x => x.Tenant)
    .Single(x => x.TrackingItemId == trackingItemId);

Enabling lazy loading would help avoid the exception if you forget to eager load the Tenant, though it has its limitations (the DbContext still needs to be in-scope) and it can result in a significant performance cost if you rely on it too much. (ie get lazy)

Instead, I highly recommend using Projection /w Select or ProjectTo (Automapper) to populate view models when reading data where you might want details from related entities. This produces more optimized queries and you don't need to worry about eager loading or tripping up on lazy loading when dealing with related entities.