Why is The navigation property '' declared on type '' has been configured with conflicting multiplicities. error show up?

Question

I have an Asp_Users table and a Resource tables. In the Entity Framework world, I have a Asp_Users POCO and a Resource POCO. Moreover, the Resource POCO is abstract and is part of a Table-per-Hierarchy model. The Table-per-Hierarchy model has the abstract Resource POCO and several Concrete POCOs like ILCResource POCO and SectionResource POCO. There is a one-to-many (1 to 0…*) relationship from Asp_Users POCO (one-side) to Resource POCO (many-side).

Here's the relevant part of my aspnet_Users POCO:

public partial class aspnet_Users
{
    public aspnet_Users() { }

    public virtual System.Guid ApplicationId { get; set; }
    public virtual System.Guid UserId { get; set; }
    public virtual string UserName { get; set; }
    public virtual string LoweredUserName { get; set; }
    public virtual string MobileAlias { get; set; }
    public virtual bool IsAnonymous { get; set; }
    public virtual System.DateTime LastActivityDate { get; set; }

    public virtual ICollection<Resource> associatedResources { get; set; }
}

Here is my mapping configuration for Asp_Users

public class Aspnet_UsersMap : EntityTypeConfiguration<PerlsData.Domain.aspnet_Users>
{
    public Aspnet_UsersMap()
    {
        this.ToTable("aspnet_Users", schemaName: "dbo");

        this.HasKey(u => u.UserId);

        this.Property(u => u.UserId)
            .HasColumnName("UserId")
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
        this.HasOptional(u => u.associatedResources);
    }
} 

Here's the relevant part of my abstract Resource POCO class:

public abstract class Resource
{   
    public Resource(){
       // associatedPerspectives = new HashSet<Perspective>();
    }

    public virtual System.Guid ResourceDatabaseID { get; set; }
    public virtual string ResourceName { get; set; }
    public virtual string DescriptionOfResource { get; set; }
    public virtual System.Guid UserId { get; set; }
    public virtual Nullable<System.Guid> DepartmentDatabaseID { get; set; }
    public virtual string ResourceStatus { get; set; }
    public virtual Nullable<short> isRemoved { get; set; }

    public virtual Department Department { get; set; }

    public virtual System.Guid UserId { get; set; }
    public virtual aspnet_Users aspnet_Users { get; set; }


    public virtual ICollection<ResourceOverToILCResourcesBridge> associatedResourceOverToILCResourcesBridgeEntry { get; set; }
}

Here is my mapping configuration for Resource:

public class ResourceMap : EntityTypeConfiguration<Resource>
{

    public ResourceMap()
    {
        this.ToTable("Resources", schemaName: "dbo");

        this.Property(r => r.ResourceDatabaseID)
            .HasColumnName("ResourceDatabaseID");

        this.HasKey(r => r.ResourceDatabaseID);

        this.Property(x => x.ResourceDatabaseID)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
        // .StoreGeneratedPattern = StoreGeneratedPattern.Identity;
        this.Property(r => r.ResourceName)
            .HasColumnName("ResourceName");

        this.Map<PerlsData.Domain.OtherItem>(m => m.Requires("discriminator").HasValue("otheritems"))
        .Map<PerlsData.Domain.Audioitem>(m => m.Requires("discriminator").HasValue("audioitems"))
        .Map<PerlsData.Domain.Imageitem>(m => m.Requires("discriminator").HasValue("imageitems"))
        .Map<PerlsData.Domain.Videoitem>(m => m.Requires("discriminator").HasValue("videoitems"))
        .Map<PerlsData.Domain.UriItem>(m => m.Requires("discriminator").HasValue("uriitems"))
        .Map<PerlsData.Domain.Documentitem>(m => m.Requires("discriminator").HasValue("documentitems"))
        .Map<PerlsData.Domain.DatabaseFileItem>(m => m.Requires("discriminator").HasValue("databasefileitems"));

        this.HasOptional(res => res.associatedResourceOverToILCResourcesBridgeEntry);

        this.HasRequired(res => res.aspnet_Users)
            .WithMany(u => u.associatedResources)
            .HasForeignKey(res => res.UserId);
    }
}

Could you please tell me why I am getting the following error?

The navigation property 'associatedResources' declared on type 'PerlsData.Domain.aspnet_Users' has been configured with conflicting multiplicities.

Please Explain Why it's still NULL after I created the mapping in the POCO class.

Solution 1

I need to study more about the IQueryable and/or IEnumerable collections along with the lambda expression queries that are used to retrieve data, and place them in the aforementioned IQueryable and/or IEnumerable.

My problem was that I was trying to retrieve the associations of a particular POCO within a loop that iterates over the IQueryable and/or IEnumerable collections.

Here is the Faulty code (we are accessing association while we iterate over IQueryable):

resourcestempIQueryable = context.Resources;
foreach (PerlsData.Domain.Resource tempRes in resourcestempIQueryable)
{
    string[] resourceRow = { tempRes.ResourceName,
                             tempRes.DescriptionOfResource,
                             tempRes.UploadDate.ToString(),
                             tempRes.Iconpath,
                             tempRes.aspnet_Users.UserName, // Error
                             tempRes.ResourceDatabaseID.ToString() };
    tempDataTableForResources.Rows.Add(resourceRow);
}

Here is the proper code that should resolve the issue:

using (PerlsData.Context context = new PerlsData.Context())
{
    context.Configuration.LazyLoadingEnabled = true;
    context.Configuration.ProxyCreationEnabled = true;
    resourcesIEnumerable = context.Resources.ToList<PerlsData.Domain.Resource>();

    var entityValuesWithFieldsToShowInAllResourcesPage = context.Resources.Select(res => new { res.ResourceName, res.DescriptionOfResource, res.UploadDate, res.Iconpath, res.aspnet_Users.UserName, res.ResourceDatabaseID });

    foreach (var varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage in entityValuesWithFieldsToShowInAllResourcesPage)
    {
        string[] resourceRow = { 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.ResourceName, 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.DescriptionOfResource, 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.UploadDate.ToString(), 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.Iconpath, 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.UserName, 
            varForIteratingOverEntityValuesWithFieldsToShowInAllResourcesPage.ResourceDatabaseID.ToString()
        };
        tempDataTableForResources.Rows.Add(resourceRow);
    }
}

Solution 2

this.HasOptional(u => u.associatedResources);

That's wrong. HasOptional means 0..1.
You want HasMany().

In fact, you can get rid of that line entirely; EF can figure it out from the property.