在代码中为实体框架定义关系 Model（未规范化）

Question

. Answers to this question are eligible for a +50 reputation bounty. Bounty grace period ends in 18 hours . Steve Olson wants to to this question:

Looking for a little help to understand how to define this relationship in code.

I am trying to force a relationship in code where it does not exist in the database. I don't have the option to redefine the database. Below are the classes generated by EF.

For each survey, there are one or form elements. That is defined in the database and comes through in the classes. For each form element, there are zero, one, or more sub elements with each type being specific to it's own table. For example, if the ElementType is 'header' than the look up needs to hit the SurveyFormHeaderElement table where the SurveyFormHeaderElement.SurveyFormHeaderElementID is equal to SurveyFormElement.ElementID. There are other tables as well for prompts and lookups with the SurveyFormElement.ElementType indicating which table to do the lookup in.

I'd like to define this in the partial classes I am using for metadata so they don't get overwritten when the model updates.

// generated code
namespace Surveys.EF.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class Survey
    {
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
        public Survey()
        {
            this.SurveyFormElements = new HashSet<SurveyFormElement>();
        }
    
        public int SurveyID { get; set; }
        public string SurveyTitle { get; set; }
    
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<SurveyFormElement> SurveyFormElements { get; set; }
    }
}

// generated code
namespace Surveys.EF.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class SurveyFormElement
    {
        public int SurveyFormElementID { get; set; }
        public int SurveyID { get; set; }
        public string ElementType { get; set; }
        public int ElementID { get; set; }
        public int ElementOrder { get; set; }
    
        public virtual Survey Survey { get; set; }
    }
}

//generated code
namespace Surveys.EF.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class SurveyFormHeaderElement
    {
        public int SurveyFormHeaderElementID { get; set; }
        public string HeaderType { get; set; }
        public string HeaderText { get; set; }
    }
}

// meta
namespace Surveys.EF.Models
{
    using System.Collections.Generic;
    using System.ComponentModel.DataAnnotations;
    
    [MetadataType(typeof(SurveyFormElementMetaData))]
    public partial class SurveyFormElement
    {
        private ICollection<SurveyFormHeaderElement> _HeaderElements;
        public virtual ICollection<SurveyFormHeaderElement> HeaderElements
        {
            get { return _HeaderElements;  }
            set { _HeaderElements = ?; } //assuming this would pull from SurveyFormHeaderElement dbcontext where SurveyFormHeaderElementID equals this.ElementID
        }
    }

    public class SurveyFormElementMetaData
    {
        
    }
}

Answer 1

You would have to build an aggregate table:

So lets say you have:

Survey PK: 1 Name "Survey 1".

And

SurveyHeaderType Pk: 1 Name: "Header"

SurveryFooterType Pk: 1 Name "Footer"

Survery_Type_Aggregate PK: 1 SurveyFooterTypeId: 1 SurveryHeaderTypeId: null

But this is only required IF you can have multiple Types on 1 survey, if you can ONLY have one type on a survey, you need to create a relationship to EACH type, and just have all but 1 be null.

So becomes

Survey PK: 1 SurveyTypeHeaderId: 1 SurveyTypeFooterId: null

Survey PK: 2 SurveyTypeHeaderId: null SurveyTypeFooterId: 1

But a relationship can only be created between two tables. By linking two values on said table.

I am not aware that it is possible to create a relationship between multiple tables where it can "option" out which table you are referencing. A relationship is always a contract between two tables.

The relationship between survey and header cannot, for example, reference footer, also.

You can create a relationship between footer and survey, but it would have to be on an independent value pair.

You can make a relationship value optional, but allowing the FK value to be null. Then that just means you have no matching row of that type. And then you have to check all references if they are not null, to find the type.

But it depends if you can have multiple types on a survey.

As far as I know, what you want to do cannot be done, because a "relationship" is limited to two tables at all times.

You can of course have multiple relationships between many, many tables. But a relationship, can only be between two tables, again, as far as I know.