CODEWITHSUNDEEP
c#entity-frameworkeager-loading
Jack Miller
Jack Miller

Reputation: 7657

EF: Eagerly include single field

I have in my database an Object which has an object Owner. Owner has a string Name. Object also holds object Details.

I need the Object, it's Details, and the Owner.Name. I want to prevent loading the whole Owner object (as detailled here) and I want only a single database query.

I tried:

DataContext.Objects.Select(o => new { Object = o, OwnerName= o.Owner.Name })
           .Include(o => o.Object.Details);

but it throws:

System.InvalidOperationException: 'The result type of the query is neither an EntityType nor a CollectionType with an entity element type. An Include path can only be specified for a query with one of these result types.

Also tried calling Include before Select:

var d = DataContext.Objects.Include(o => o.Details)
.Select(o => new { Object = o, OwnerName= o.Owner.Name });

But this ignores the Include, ie. Accessing d.Details causes a DB query.

Any ideas?

Upvotes: 1

Views: 1102

Answers (2)

Flater
Flater

Reputation: 13783

You're touching on an issue that does exist in EF.

The problem lies in how EF handles the loading of data. It loads all scalar properties of an object, but not the navigational properties.

Include influences this behavior, by telling EF to also include a specified navigational property (with all of its scalar properties)

But then we get to Select. When you use this, you are essentially giving a fixed list of columns that you want to retrieve. This overrides the default behavior of loading all scalar properties, which sadly also overrides the Include statements that you added.

The simplest solution is to explicitly tell EF to retrieve the details:

var d = DataContext.Objects.Include(o => o.Details)
    .Select(o => new { 
               Object = o, 
               ObjectDetails = o.Details,
               OwnerName= o.Owner.Name 
    });

As an aside, since the Select explicitly states which columns EF needs to retrieve, you don't need the Include statement anymore.

This behavior can already be seen by you retrieving o.Owner.Name without actually calling Include(o => o.Owner).

Upvotes: 0

Bradley Uffner
Bradley Uffner

Reputation: 16991

This should do it (though Details are loaded in a separate property to correct for @Flatter's comment):

DataContext.Objects
           .Include(o => o.Owner>)
           .Include(o => o.Details)
           .Select(o => new {Object = o, 
                             OwnerName = o.Owner.Name,
                             Details = o.Details});

It returns a new object with Object set to the selected Object (consider renaming this if you can, its very confusing), and an OwnerName populated from the Owner table without loading the entire Owner. It also populates Details as a separate property, since Select ignores the Include.

I say it "should" work, because without seeing your table structure, I have to guess based on your descriptions of how things are connected.

Upvotes: 0

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