如何在实体框架 6 中动态加载子实体

Question

I have a Provider class and an Article one. Article has a int ProviderId{get;set} and a public virtual Provider Provider {get;set;} properties.

I know about Lazy loading and why I can't access the Provider property in Article outside the context but I have a generic method that returns the next T like this:

public static T Next<T>(T currentElement) where T : class, IModel {
    T data;

    if (currentElement.Id >= GetLastId<T>())
        return currentElement;

    using (DatabaseEntities context = new DatabaseEntities()) {
        data = context.Set<T>().Single(el => el.Id == currentElement.Id + 1);
    }

    return data;
}

But I can't retrieve Child entities like Provider in Articles Class. How can I include all entities? Should I update the method and make one per entity?

I read about Eager loading and Explicit loading but I don't know how can I implement these in my method.

Note:

Not all my entities have entity children and I have more methods like Previous<T>() , First<T>() or Last<T>() that do the work you expect.

Answer 1

You could create an overload of your Next method that accepts an Expression<Func<T, object>> :

public static T Next<T>(T currentElement, Expression<Func<T, object>> navigationProperty) where T : class, IModel
{
    T data;

    if (currentElement.Id >= GetLastId<T>())
        return currentElement;

    using (DatabaseEntities context = new DatabaseEntities())
    {
        IQueryable<T> dbQuery = context.Set<T>();
        if (navigationProperty != null)
            dbQuery = dbQuery.Include<T, object>(navigationProperty);

        data = dbQuery.AsNoTracking().Single(el => el.Id == currentElement.Id + 1);
    }
    return data;
}

Usage:

var entity = Next(instance, x => x.Provider);

Please refer to the following blog post for more information and a complete example.

Implementing a generic data access layer using Entity Framework: https://blog.magnusmontin.net/2013/05/30/generic-dal-using-entity-framework/

Answer 2

You could extend your method to take a list of possible includes. For example:

public static T Next<T>(T currentElement, params Expression<Func<T, object>>[] includes)
    where T : class, IModel
{
    if (currentElement.Id >= GetLastId<T>())
        return currentElement;

    using (DatabaseEntities context = new DatabaseEntities())
    {
        IQueryable<T> query = context.Set<T>();

        //Deferred execution allows us to build up the query
        foreach (var include in includes)
        {
            query = query.Include(include);
        }

        return query.Single(el => el.Id == currentElement.Id + 1);
    }
}

And use it like this:

var someEntity = ...;

var nextEntity = Next(someEntity, 
    e => e.Childcollection1,
    e => e.Childcollection2,
    //
    e => e.ChildcollectionN)

As an additional point, to get your next entity, you shouldn't rely on the ID values being sequential, for example, they will get out of sequence if entries are deleted. Instead, consider something like this:

return query.OrderBy(el => el.Id).First(el => el.Id > currentElement.Id);

Answer 3

So you have a one-to-many relation between Provider and Article : Every Provider has zero or more Articles , and every Article belongs to exactly one Provider .

If you have configured this one-to-many relationship correctly in Entity Framework, a Provider should have a reference to its many Articles :

public virtual ICollection<Article> Articles{get; set;}

Furthermore you have a function Next , that takes as input an object of class T , which is a class that implements IModel . Next returns the next element from the DbSet of Ts, assuming you have a proper definition for the next element.

Apparently you want to adapt function Next such that you can use this function to get the next Provider with all its Articles .

More generic: if T is a type that has properly designed one-to-many relationships in it, and you have an object of type T, you want the T with the first Id higher than the Id of the current T, inclusive all its ICollections.

I've divided this into sever smaller functions:

• A function that, given a type T returns all properties that implement ICollection
• A function that, given a DbSet returns all elements of the DbSet inclusive all its ICollections

Given those two, and functions like OrderBy and FirstOrDefault you will be able to get the first Provider with Id larger than current provider with all its Articles.

static class QueryableExtensions
{
    // returns true if PropertyInfo implements ICollection<...>
    public static bool ImplementsICollection(this PropertyInfo propertyInfo)
    {
         return propertyInfo.IsGenericType
             && propertyInfo.GetGenericTypeDefinition() == typeof(ICollection<>);
    }

    // returns all readable & writable PropertyInfos of type T that implement ICollection<...>
    public static IEnumerable<PropertyInfo> CollectionProperties<T>()
    {
        return typeof(T).GetProperties()
            .Where(prop => prop.CanRead
                && prop.CanWrite
                && prop.PropertyType.ImplementICollection();
    }

    // given an IQueryable<T>, adds the Include of all ICollection<...> T has
    public static IQueryable<T> IncludeICollection<T>(IQueryable<T> query)
    {
         var iCollectionProperties = CollectionProperties<T>();
         foreach (var collectionProperty in collectionProperties)
         {
              query = query.Include(prop.Name);
         }
         return query;
    }

    // given a IQueryable<T> and a T return the first T in the query with Id higher than T
    // for this we need to be sure every T has an IComparable property Id
    // so T should implement interface IId (see below)
    public T Next<T>(this IQueryable<T> query, T currentItem)
         where T : IId  // makes sure every T has aproperty Id
    {
         T nextElement = query
             // keep only those DbSet items that have larger Ids:
             .Where(item => currentItem.Id < item.Id)
             // sort in ascending Id:
             .OrderBy(item => item.Id
             // keep only the first element of this sorted collection:
             .FirstOrDefault();
        return T
    }
}

I need to be sure that every T has an Id, otherwise you can't get the next Id. Probably you have this in your IModel interface:

public Interface IId
{
     public int Id {get;}
}

After these functions your query will be like:

public static T Next<T>(T currentElement) where T : class, IModel, IId
{
    T data;
    if (currentElement.Id >= GetLastId<T>())
        return currentElement;

    using (DatabaseEntities context = new DatabaseEntities())
    {
        return context.Set<T>().Next(currentElement);
    }
 }

Answer 4

I'm not looking to rehash an answer, but here is a function that combines a little bit of reflection to automagically load all foreign key related data based on model declarations. I've written it as a way to load an object by something other than its primary key, but it can be modified to do so if needed.

 private IQueryable<T> GetFullSet()
    {
        IEnumerable<IEntityType> entities = _dbContext.Model.GetEntityTypes();
        IEntityType thisEntity = entities.SingleOrDefault(x => x.ClrType.Name == typeof(T).Name);

        IQueryable<T> set = _dbContext.Set<T>();

        foreach (IForeignKey fKey in thisEntity.GetReferencingForeignKeys())
        {
            set = set.Include(fKey.PrincipalToDependent.Name);
        }

        return set;
    }