实体框架映射多级属性

Question

I am trying to implement a hierarchical inheritance structure in Entity Framework, specifically for settings. For example, lets say we have user preferences:

public class StorePreference: Preference { }

public class UserPreference : Preference { }

public class Preference {
public string BackgroundColor { get; set; }
public ContactMethod ContactMethod { get; set; }
}

public enum ContactMethod {
  SMS,
  Email
}

I'd like it so that if I lookup the user's preferences. If the user doesn't exist or the property value is null, it looks up the parent (store) default preferences.

Ideally, i'd like it to work similar to abstract inheritance:

public class UserPreference : StorePreference {
    private string _backgroundColor;

    public string BackgroundColor {
        get { 
           if (this._backgroundColor == null)
              return base;

           return this._backgroundColor;
        } 
        set { this._backgroundColor = value; }
    }
}

If I were to write this as a SQL query, it'd be a CROSS APPLY with a CASE statement:

SELECT 
    CASE WHEN User.BackgroundColor == null THEN Store.BackgroundColor ELSE User.BackgroundColor END BackgroundColor,
    CASE WHEN User.ContactMethod == null THEN Store.ContactMethod ELSE User.ContactMethod END ContactMethod
FROM UserPreference User
CROSS APPLY StorePreference Store
WHERE UserPreference.UserId = @UserId

Is there a way I can achieve loading this in EF?

Answer 1

In your base class add default property values:

public class Preference {
    public string BackgroundColor { get; set; } = "Red";
    public ContactMethod ContactMethod { get; set; } = ContactMethod.SMS;
}

Something like this to set from database:

public class StorePreference : Preference { }

public class UserPreference : Preference { }

public class Preference {
    public Preference() {
        BackgroundColor = DefaultPreference.BackgroundColor;
        ContactMethod = DefaultPreference.ContactMethod;
    }

    public string BackgroundColor { get; set; }
    public ContactMethod ContactMethod { get; set; }

    public DefaultPreference DefaultPreference { get; set; }
}

public class DefaultPreference {
    public string BackgroundColor { get; set; }
    public ContactMethod ContactMethod { get; set; }
}

Answer 2

As long as the properties are public, entity won't have a problem pulling the data from another table as the default. You would need to create a private field to hold the data if you used a setter:

public class ChildTable : EntityBase {
    private string _someCategory;

    [Key]
    [Column(name: "CHILD_ID")]
    public override int Id { get; protected set; }

    [Column(name: "SOME_CATEGORY")]
    public string SomeCategory {
        get { return _someCategory; }
        set { _someCategory = value ?? ParentTable.SomeCategory; }
    }

    [ForeignKey("ParentTable")]
    [Column(name: "PARENT_ID")]
    public int ParentTableId { get; set; }

    public virtual ParentTable ParentTable { get; set; }
}

This is just an alternative to a constructor, if you need more control over the setter logic, otherwise Austin's answer would be simpler to implement