LINQ query cannot be translated if DbContext is wrapped in an interface

Question

I'm having a nasty problem I really can't understand.

Basically I have a DbContext defined as follows

public interface ISettingsContext : IDisposable 
{
    IDbSet<Site> Sites { get; }
    IDbSet<SettingGroup> Groups { get; }
    IDbSet<SettingProperty> Properties { get; }

    int SaveChanges();
}

public class SettingsContext : DbContext, ISettingsContext
{
    public SettingsContext(string connectionStringName) : base(connectionStringName)
    {
        Sites = Set<Site>();
        Groups = Set<SettingGroup>();
        Properties = Set<SettingProperty>();
    }

    public SettingsContext() : this("DefaultTestConnectionString") { }

    public IDbSet<Site> Sites { get; private set; }

    public IDbSet<SettingGroup> Groups { get; private set; }

    public IDbSet<SettingProperty> Properties { get; private set; }

    #region Model Creation

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        MapSite(modelBuilder.Entity<Site>());
        MapGroup(modelBuilder.Entity<SettingGroup>());
        MapProperty(modelBuilder.Entity<SettingProperty>());
        MapValue(modelBuilder.Entity<SiteSettingValue>());
        MapXmlValue(modelBuilder.ComplexType<XmlValue>());
        MapXmlType(modelBuilder.ComplexType<XmlTypeDescriptor>());
    }

    private void MapXmlType(ComplexTypeConfiguration<XmlTypeDescriptor> complexType)
    {
        complexType.Ignore(t => t.Type);
    }

    private void MapXmlValue(ComplexTypeConfiguration<XmlValue> complexType)
    {
        complexType.Ignore(t => t.Value);
    }

    private void MapValue(EntityTypeConfiguration<SiteSettingValue> entity)
    {
        entity.HasKey(k => new { k.PropertyId, k.SiteId }).ToTable("SiteValues", "settings");

        entity.Property(k => k.PropertyId).HasColumnName("FKPropertyID");

        entity.Property(k => k.SiteId).HasColumnName("FKSiteID");

        entity.Property(k => k.Value.RawData).HasColumnName("Value").IsMaxLength();

        entity.HasRequired(k => k.Site).WithMany(s => s.Settings).HasForeignKey(k => k.SiteId);

        entity.HasRequired(k => k.Property).WithMany().HasForeignKey(k => k.PropertyId);
    }

    private void MapProperty(EntityTypeConfiguration<SettingProperty> entity)
    {
        entity.HasKey(k => k.Id).ToTable("Properties", "settings");

        entity.Property(k => k.Id);

        entity.Property(k => k.Name);

        entity.Property(k => k.GroupId).HasColumnName("FKGroupID");

        entity.Property(k => k.PropertyDescriptor.RawData).HasColumnName("TypeDescriptor").IsMaxLength();

        entity.Property(k => k.DefaultValue.RawData).HasColumnName("DefaultValue").IsMaxLength();

        entity.HasRequired(k => k.Group).WithMany(k => k.Properties).HasForeignKey(k => k.GroupId);
    }

    private void MapGroup(EntityTypeConfiguration<SettingGroup> entity)
    {
        entity.HasKey(k => k.Id).ToTable("Groups", "settings");

        entity.Property(k => k.Id);

        entity.Property(k => k.Name);

        entity.HasMany(k => k.Properties).WithRequired(k => k.Group).HasForeignKey(k => k.GroupId);
    }

    private void MapSite(EntityTypeConfiguration<Site> entity)
    {
        entity.HasKey(k => k.Id).ToTable("Sites", "site");

        entity.Property(k => k.Id);

        entity.Property(k => k.Domain);
    }

    #endregion
}

Since I was carrying on some tests, I started working on this context without caring about IoC and DI, so I worked directly against the concrete class itself.

So I wrote this query:

public class SiteSettingsLoader : ILoader<SiteSettingsModel, int> {
...
var qSiteValues = from site in context.Sites
                  let settings = site.Settings
                  select new
                  {
                          SiteId = site.Id,
                          Settings = from value in settings
                                     join property in context.Properties on value.PropertyId equals property.Id into props
                                     from property in props
                                     select new
                                     {
                                             PropertyId = property.Id,
                                             Name = property.Name,
                                             GroupId = property.GroupId,
                                             Value = value.Value,
                                             CanBeInherited = property.CanBeInherited
                                     }
                  };

And everything worked just fine. The problems started when I started passing the context as the interface it implements instead of its real type.

using (ISettingsContext context = new SettingsContext())
using (SiteSettingsLoader loader = new SiteSettingsLoader(context))
{
    SiteSettingsStore store = new SiteSettingsStore(loader);
    dynamic settings = store.GetItem(1);
}

This is the error

Unable to create a constant value of type 'Settings.Entities.SettingProperty'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

How is it possible? Did Microsoft play some dirty trick with reflection?

EDIT: Both XmlValue and XmlTypeDescriptor are an implementation of the following interface that we use to automatically read the content of an XML field into a proper object.

public interface IXmlProperty
{
    string RawData { get; set; }
    void Load(string xml);
    XDocument ToXml();
}

Basically we map the content of the field to the RawData property; in its setters and getters we parse/generate the string containing the XML for the database. I know it's an ugly trick but it works and gets the job done.

Answer 1

Based on this answer to a very similar question the reason for the problem seems to be that EF does not understand the properties of your custom interface type as queryable expressions and considers the context.Properties collection as a collection of constant objects of type SettingsProperty that you want to pass into the query. Using non-primitive constant objects is not supported by EF which causes your exception.

Basically you need to use a (compile time) type that EF knows how to translate into a SQL fragment to incorporate into the whole query. That would be a DbContext.DbSet<T> itself or an IQueryable<T>:

IQueryable<SettingProperty> properties = context.Properties;
var qSiteValues = from site in context.Sites
                  let settings = site.Settings
                  select new
                  {
                      SiteId = site.Id,
                      Settings = from value in settings
                                 join property in properties
                                    on value.PropertyId equals property.Id
                                    into props
                                 from property in props
                                 select new
                                 {
                                     PropertyId = property.Id,
                                     Name = property.Name,
                                     GroupId = property.GroupId,
                                     Value = value.Value,
                                     CanBeInherited = property.CanBeInherited
                                 }
              };

It might be necessary to use the same trick for context.Sites but I am not sure.

(If the above works don't forget to upvote Eranga's linked answer because it's awesome.)