使用 Ef Core 选择分层数据中的特定列

Question

I want to use the functionality of EF Core to automatically map self references to child lists even when I load a flat structure. In addition, I want to be able to reduce the number of columns read and transferred.

My self referencing data structure:

public class Item 
{
    public int property1 { get; set; }
    /*
        Realy long list of columns
    */
    public int property30 { get; set; }

    public Guid? ParentOid { get; set; }
    public virtual Item Parent { get; set; }
    public virtual ICollection<OtherObject> OtherObjects { get; set; }
    public virtual ICollection<Item> Children { get; set; }
}

I want to achieve loading of the whole hierarchical Tree which can be done easily by using the following query:

(DbContext context) =>  context.Items.AsQueryable()

And afterwards filtering with:

var rootNodes = items.Where(f => f.ParentOid == null)

This way Ef Core already filled the Children collection on all Children in the tree, this is the behavior that I need!

This query transfers quite huge amounts of data because property1 - property30 are all loaded while I just need a small subset of them.

By using the LINQ select statement with anonymous types, I'm loosing the behavior of building up the tree automatically and I would have to do that by myself.

My current solution

By setting up another model Item.cs without the properties, I don't need their presence in the select statement of the SQL query.

My problem with this solution is that I need to change the name of Item.cs to eg ItemWithLessProperties.cs and add it to the current context. This would create a conflict in OtherObject.cs because its holding references of Item and not ItemWithLessProperties. So, I would have to copy this one and add it to the context as well.

By using a total different context for this purpose, I would also have duplicates of the Models Item (which is not a big problem) and OtherObject (which is a problem, because there are multiple of those).

Answer 1

Sounds like you need an ItemDTO . You can create an object like

public class ItemDTO 
{
    /*
        Properties you need
    */
    public int property1 { get; set; }
    public int property5 { get; set; }
    public int property17 { get; set; }
    public int property30 { get; set; }
    public virtual ICollection<OtherObject> OtherObjects { get; set; }
    public virtual ICollection<Item> Children { get; set; }
}

And also for every nested entity a DTO if you want to also include it in the queryable.

Then you can simply var dtoItems = context.Items.Where(condition).Select(i=>new ItemDTO {/* initialize props */});

Note : If you are not using a DTO and if the data you pull would be strictly read only and you won't try to update the currently pulled entries in the database it's a good option to also use .AsNoTracking() to avoid the change tracking overhead of EF Core.