How can I find the Id property or properties relat

问题:

For a project I'm working with Entity Framework and I'd like to be able to enumerate all navigational properties for a given object instance (assuming it's an object generated by EF). From there I'd like to get the related Id property for every navigational property.

For example, if I get an instance of the class Person, I want to be able to find it's navigational properties called Address and Boss. For those two navigational properties I want to then "lookup" the related Id properties called AddressId and BossId.

I need those Id properties so I can run queries on a different database which does not have the same foreign keys but does have exactly the same Ids.

So far I have figured out a way to get the RelationshipManager for a random object instance generated by EF. And while debugging I can get to the foreign key relations via the Manager's Relationships property. But I can only get as far as the navigational property name. So I can see there's a FK_Person_Address which is related to the navigational property called Address but I can't find the AddressId.

So my question is, how can I dynamically (with no knowledge of the Person class' layout) discover the AddressId property which is related to Address?

I am aware the Foreign Key relationship might have the Id property on the other side of the relation (Boss pointing to Person in stead of Person having a BossId). In that case, I'd still like to discover that Boss has a PersonId when I'm inspecting an instance of Person.

回答1:

This will give you a dictionary with all navigation properties as Key and all related properties as Value (the value might be a property from the other entity)

Add these to your DBContext class and call db.GetForeignKeyProperties<Person>()

The result will be something like:

"Address" - "AddressID"

"Boss" - "Person.BossID"

public Dictionary<string,string> GetForeignKeyProperties<DBType>()
{
    EntityType table = GetTableEntityType<DBType>();
    Dictionary<string, string> foreignKeys = new Dictionary<string, string>();

    foreach (NavigationProperty np in table.NavigationProperties)
    {
        var association = (np.ToEndMember.DeclaringType as AssociationType);
        var constraint = association.ReferentialConstraints.FirstOrDefault();



        if (constraint != null && constraint.ToRole.GetEntityType() == table)
            foreignKeys.Add(np.Name, constraint.ToProperties.First().Name);

        if (constraint != null && constraint.FromRole.GetEntityType() == table)
            foreignKeys.Add(np.Name, constraint.ToProperties.First().DeclaringType.Name+"."+constraint.ToProperties.First().Name);
    }

    return foreignKeys;
}

private EntityType GetTableEntityType<DBType>()
{
    return GetTableEntityType(typeof(DBType));
}

private EntityType GetTableEntityType(Type DBType)
{
    ObjectContext objContext = ((IObjectContextAdapter)this).ObjectContext;
    MetadataWorkspace workspace = objContext.MetadataWorkspace;
    EntityType table = workspace.GetEdmSpaceType((StructuralType)workspace.GetItem<EntityType>(DBType.FullName, DataSpace.OSpace)) as EntityType;
    return table;
}

回答2:

Here's a method that returns key values of know entity objects:

IEnumerable<IDictionary<string,object>> GetKeyValues<T>(DbContext db, 
                                                        IEnumerable<T> entities)
    where T : class
{
    var oc = ((IObjectContextAdapter)db).ObjectContext;
    return entities.Select (e => oc.ObjectStateManager.GetObjectStateEntry(e))
                   .Select(objectStateEntry => objectStateEntry.EntityKey)
                   .Select(ek => ek.EntityKeyValues
                                   .ToDictionary (x => x.Key, y => y.Value));
}

This method uses the underlying ObjectContext API to obtain ObjectStateEntry objects that belong to each entity object. The EntityKey contains the key value(s) of an entity as a key-value pair. (Entities with composite keys have more than one key value.)