Entity Framework code first. Find primary key

Question

How do I find which property of a class is the primary key of the Entity Framework Code First entity POCO?

Please note string matching for Id / class name + "Id" is a bad option. There must be some way to dig out the convention used by Entity Framework and reliably getting the key property.

Thanks in advance.

score 62 · Accepted Answer · answered Aug 31 '11 at 07:45

62

You can ask mapping metadata to get names of key properties (there can be more then one):

ObjectContext objectContext = ((IObjectContextAdapter)dbContext).ObjectContext;
ObjectSet<YourEntity> set = objectContext.CreateObjectSet<YourEntity>();
IEnumerable<string> keyNames = set.EntitySet.ElementType
                                            .KeyMembers
                                            .Select(k => k.Name);

Once you have key names you can use reflection to access their values.

As you can see the approach reverts back to ObjectContext API because DbContext API is only for simple scenarios where you don't bother with such details like mapping metadata.

answered Aug 31 '11 at 07:45

Ladislav Mrnka

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Thx. marked as answer. I do like to comment that It's a better idea to add an interface to your context class (eg, IYourDataContext) and inherit the IObjectContextAdapter to that IYourDataContext. That way you don't have to cast it. – Sleeper Smith Sep 01 '11 at 06:10
Given this is 5 old now, is there a way to do this without heavy reflection? The context of my question is an attempt to implement a Find method without tracking, such as SingleOrDefault() but in a base class that cannot assume to know the entity's key name. Thanks guys. – one.beat.consumer Jan 03 '17 at 19:46
Instead of finding the primary key, is there a way to figure out which properties have (unique) indexes on them? – Rudey Apr 12 '19 at 18:54

score 28 · Answer 2 · answered Aug 20 '14 at 10:01

In case it helps anyone, I needed to be able to do this without the knowing the type beforehand (so I couldn't easily do CreateObjectSet<YourEntity>() because I didn't know YourEntity), so I was able to adapt @Ladislav 's solution into the following:

// variable "type" is a System.Type passed in as a method parameter
ObjectContext objectContext = ((IObjectContextAdapter)this.context).ObjectContext;
IEnumerable<string> retval = (IEnumerable<string>)objectContext.MetadataWorkspace
    .GetType(type.Name, type.Namespace, System.Data.Entity.Core.Metadata.Edm.DataSpace.CSpace)
    .MetadataProperties
    .Where(mp => mp.Name == "KeyMembers")
    .First()
    .Value;

Seems kind of odd that MetadataWorkspace.GetType requires strings of the type name and namespace, instead of a System.Type, but that's the best I could find.

Instead of finding the primary key, is there a way to figure out which properties have (unique) indexes on them? — Rudey, Apr 12 '19 at 18:54

angularsen · Answer 3 · 2016-06-15T08:06:54.843

14

In EF 6.1 there is a Db() extension method that makes this easier.

Nuget: https://www.nuget.org/packages/EntityFramework.MappingAPI/ (v6.1.0.9)
Namespace: EntityFramework.MappingAPI.Extensions.MappingApiExtensions

Example:

public static IEnumerable<string> GetPrimaryKeyPropertyNames(DbContext db, Type entityType)
{
    return db.Db(entityType).Pks.Select(x => x.PropertyName);
}

edited Jun 15 '16 at 08:06

answered Jun 14 '16 at 13:18

angularsen

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How's its performance though? Heavy with underlying reflection? – one.beat.consumer Jan 03 '17 at 19:54
1

I have no idea, EntityFramework is to my knowledge based on a lot of reflection to begin with. – angularsen Jan 04 '17 at 12:13
3

The linked package is obsolete and is no longer listed on NuGet – Aaroninus Apr 20 '17 at 17:59
Thanks for pointing that out, it's likely obsoleted by a newer version of EF. – angularsen Apr 21 '17 at 07:54
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@anjdreas Or it's because it [now costs $700](http://entityframework-extensions.net/?z=nuget&y=mappingapi). – John Jun 23 '17 at 10:58
Wow, that's.. more than I'm willing to pay for this usecase alone :-) – angularsen Jun 23 '17 at 21:25

score 9 · Answer 4 · answered Sep 18 '14 at 15:51

I had problem with both the above approaches because of Table Per Type inheritance. My working version (based on @S'pht'Kr's solution but using DataSpace.OSpace not DataSpace.CSpace for this reason) is below:

        protected IEnumerable<string> GetKeyPropertyNames()
        {
            var objectContext = ((System.Data.Entity.Infrastructure.IObjectContextAdapter) this.Context).ObjectContext;

            return GetKeyPropertyNames(typeof (TEntity), objectContext.MetadataWorkspace);
        }

        private static IEnumerable<string> GetKeyPropertyNames(Type type, MetadataWorkspace workspace)
        {
            EdmType edmType;

            if (workspace.TryGetType(type.Name, type.Namespace, DataSpace.OSpace, out edmType))
            {
                return edmType.MetadataProperties.Where(mp => mp.Name == "KeyMembers")
                    .SelectMany(mp => mp.Value as ReadOnlyMetadataCollection<EdmMember>)
                    .OfType<EdmProperty>().Select(edmProperty => edmProperty.Name);
            }

            return null;
        }

score 4 · Answer 5 · answered Jun 27 '16 at 15:20

Like user rashleighp, I also wanted a variant of user Ladislav Mrnka's answer that only needs to know the type at runtime instead of needing to know the type at compile time. Also like user rashleighp, the solution of user S'pht'Kr didn't work for me, but his solution did work. Below, I contribute to this conversation by providing a simpler version of his answer that worked for me. However, I just learned about the solution by user anjdreas and that is the one that I will be using.

// variable "type" is a System.Type passed in as a method parameter
((IObjectContextAdapter)context)
    .ObjectContext
    .MetadataWorkspace
    .GetItem<EntityType>(type.FullName, DataSpace.OSpace)
    .KeyProperties
    .Select(p => p.Name);

Entity Framework code first. Find primary key

5 Answers5

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