Implementing Missing Features in Entity Framework Core – Part 7: Entity Configuration in Mapping Classes

This is the seventh post in a series of posts about bringing the features that were present in Entity Framework pre-Core into EF Core. The others are:

  • Part 1: Introduction, Find, Getting an Entity’s Id Programmatically, Reload, Local, Evict

  • Part 2: Explicit Loading

  • Part 3: Validations

  • Part 4: Conventions

  • Part 5: Getting the SQL for a Query

  • Part 6: Lazy Loading

This time I’m going to cover automatic entity configuration in mapping classes, that is, outside of the DbContext.OnModelCreating method. If you remember, Entity Framework Code First supported having classes inheriting from EntityTypeConfiguration in the same assembly as the context, and these classes would be loaded automatically. This made it much simpler to add new mapping classes to a project without touching the context.

This functionality hasn’t been ported to Entity Framework Core yet, but it is being developed for the next version, and is tracked by ticket 2805. My implementation finds mapping classes in the same assembly as the context automatically, or it can load configuration explicitly from a mapping class. Here is my contract for the mapping class:

public interface IEntityTypeConfiguration<T> where T : class
{
void Configure(EntityTypeBuilder<T> entityTypeBuilder);
}

As you can see, this needs to be implemented in a concrete generic class and bound to a specific entity type.

The actual implementation goes like this:

public static class EntityTypeConfigurationExtensions
{
private static readonly MethodInfo entityMethod = typeof(ModelBuilder).GetTypeInfo().GetMethods().Single(x => (x.Name == "Entity") && (x.IsGenericMethod == true) && (x.GetParameters().Length == 0));

private static Type FindEntityType(Type type)
{
var interfaceType = type.GetInterfaces().First(x => (x.GetTypeInfo().IsGenericType == true) && (x.GetGenericTypeDefinition() == typeof(IEntityTypeConfiguration<>)));
return interfaceType.GetGenericArguments().First();
}

private static readonly Dictionary<Assembly, IEnumerable<Type>> typesPerAssembly = new Dictionary<Assembly, IEnumerable<Type>>();

public static ModelBuilder ApplyConfiguration<T>(this ModelBuilder modelBuilder, IEntityTypeConfiguration<T> configuration) where T : class
{
var entityType = FindEntityType(configuration.GetType());

dynamic entityTypeBuilder = entityMethod
.MakeGenericMethod(entityType)
.Invoke(modelBuilder, new object[0]);

configuration.Configure(entityTypeBuilder);

return modelBuilder;
}

public static ModelBuilder UseEntityTypeConfiguration(this ModelBuilder modelBuilder)
{
IEnumerable<Type> configurationTypes;
var asm = Assembly.GetEntryAssembly();

if (typesPerAssembly.TryGetValue(asm, out configurationTypes) == false)
{
typesPerAssembly[asm] = configurationTypes = asm
.GetExportedTypes()
.Where(x => (x.GetTypeInfo().IsClass == true) && (x.GetTypeInfo().IsAbstract == false) && (x.GetInterfaces().Any(y => (y.GetTypeInfo().IsGenericType == true) && (y.GetGenericTypeDefinition() == typeof(IEntityTypeConfiguration<>)))));
}

var configurations = configurationTypes.Select(x => Activator.CreateInstance(x));

foreach (dynamic configuration in configurations)
{
ApplyConfiguration(modelBuilder, configuration);
}

return modelBuilder;
}
}

You can see that it uses some dynamic magic to make things simpler, otherwise we’d need to have even more reflection. Dynamics take care of these things quite nicely.

The code essentially looks at the entry assembly and finds all non-abstract public types that implement IEntityTypeConfiguration<T>. For each of those, it creates an instance, extracts the template argument and creates an EntityTypeBuilder<T> from calling the Entity<T> method of the ModelBuilder class and calls the IEntityTypeConfiguration<T>.Configure method of the instantiated mapping class passing it the EntityTypeBuilder<T> which allows it to supply mapping configuration for the mapped entity (T).

We need to explicitly call this extension inside DbContext.OnModelCreating:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.UseEntityTypeConfiguration();
base.OnModelCreating(modelBuilder);
}

And it takes care of everything for us. Or, if we want to load a single mapping class explicitly, we can also do so:

modelBuilder.UseEntityTypeConfiguration<MyEntityTypeConfiguration>();

Finally, a simple mapping class might be:

public class MyEntityTypeConfiguration : IEntityTypeConfiguration<MyEntity>
{
public void Configure(EntityTypeBuilder<MyEntity> entityTypeBuilder)
{
entityTypeBuilder.ToTable("MyEntity");
entityTypeBuilder.Property(x => x.MyEntityId).HasColumnName("Id");
}
}

In case you’re interested, this feature is similar to the one being implemented for .NET Core, except that it doesn’t find mapping classes automatically. The IEntityTypeConfiguration<T> interface is exactly the same.

Entity Framework Pitfalls: Skip/Take Position Matters

In LINQ, you normally do paging through the Skip and Take operators. These are the LINQ counterparts to whatever the relational database uses for pagination – ROW_NUMBER(), LIMIT…OFFSET, TOP, ROWNUM, etc.

There are some gotchas to it:

  • When you do paging, Entity Framework demands that you add ordering to your query. This makes sense as without an explicit ORDER BY clause, search results may come in an unexpected order;
  • Skip and Take yield different results if added to different places in the LINQ expression.

Consider these two LINQ queries:

ctx
.MyEntities
.Where(x => x.Name == "")
.OrderBy(x => x.Id)
.Skip(10)
.Take(5)
.ToList();

And:

ctx
.MyEntities
.OrderBy(x => x.Id)
.Skip(10)
.Take(5)
.Where(x => x.Name == "")
.ToList();

The first one will produce this SQL:

SELECT TOP (5)
[Filter1].[Id] AS [Id],
[Filter1].[Name] AS [Name],
[Filter1].[Date] AS [Date]
FROM ( SELECT [Extent1].[Id] AS [Id], [Extent1].[Name] AS [Name], [Extent1].
[Date] AS [Date], row_number() OVER (ORDER BY [Extent1].[Id] ASC) AS [row_number
]
FROM [dbo].[MyEntities] AS [Extent1]
WHERE N'' = [Extent1].[Name]
) AS [Filter1]
WHERE [Filter1].[row_number] > 10
ORDER BY [Filter1].[Id] ASC

Whereas the second will produce this one:

SELECT
[Limit1].[Id] AS [Id],
[Limit1].[Name] AS [Name],
[Limit1].[Date] AS [Date]
FROM ( SELECT TOP (5) [Extent1].[Id] AS [Id], [Extent1].[Name] AS [Name], [Extent1].[Date] AS [Date]
FROM ( SELECT [Extent1].[Id] AS [Id], [Extent1].[Name] AS [Name], [Extent1].[Date] AS [Date], row_number() OVER (ORDER BY [Extent1].[Id] ASC) AS [row_number]
FROM [dbo].[MyEntities] AS [Extent1]
) AS [Extent1]
WHERE [Extent1].[row_number] > 10
ORDER BY [Extent1].[Id] ASC
) AS [Limit1]
WHERE N'' = [Limit1].[Name]
ORDER BY [Limit1].[Id] ASC

Basically, the TOP(5) expression is placed in the wrong place. So, in LINQ, it’s not just the presence of a Skip and Take operator, it’s where you place it that matters!

Entity Framework Pitfalls: Null Semantics

Imagine you have a simple class:

public class MyEntity
{
public int Id { get; set; }
public string Name { get; set; }
}

You may want to query it:

string name = ...;

var entities = ctx
.MyEntities
.Where(x => x.Name == name)
.ToList();

Entity Framework generates a query such as this:

SELECT [Extent1].[Id] AS Id, [Extent1].[Name] AS Name
FROM [dbo].[MyEntities] AS [Extent1]
WHERE ([Extent1].[Name] = @p__linq__0)
OR (([Extent1].[Name] IS NULL) AND (@p__linq__0 IS NULL))

Meaning: return all records from the MyEntities table if

  1. The Name column is identical to the name variable or
  2. Both the Name column and the name variable are NULL

Seems excessively complex, doesn’t it? Well, this is to account for the case where the name parameter is null. In  that case, a simple COLUMN = VALUE comparison won’t work, because in SQL, NULL = NULL returns FALSE! The syntax for comparing with NULL is COLUMN IS NULL. By generating this kind of query, Entity Framework is playing safe. Mind you, this only happens if you do not use the literal null, if you do, Entity Framework is smart enough to generate a simpler query:

var entities = ctx
.MyEntities
.Where(x => x.Name == null)
.ToList();

Results in:

SELECT [Extent1].[Id] AS Id, [Extent1].[Name] AS Name
FROM [dbo].[MyEntities] AS [Extent1]
WHERE ([Extent1].[Name] IS NULL)

However, there is a configuration setting, UseDatabaseNullSemantics (the opposite of the old UseCSharpNullComparisonBehavior of ObjectContext), that can be used to control this behavior. If set to true (the opposite of the default):

ctx.Configuration.UseDatabaseNullSemantics = true;

It generates all SQL queries as the unsafe version:

SELECT [Extent1].[Id] AS Id, [Extent1].[Name] AS Name
FROM [dbo].[MyEntities] AS [Extent1]
WHERE ([Extent1].[Name] = @p__linq__0)

The result is, even if you have a record whose Name column is NULL, it won’t be returned: remember that in SQL, by default, NULL <> NULL!

Therefore, if you are not 100% sure that the parameter that you will be using in the LINQ query will never be null, do not mess with UseDatabaseNullSemantics even if it generates somewhat poorer SQL.

Of course, if you are using SQL Server, you can set ANSI_NULLS to OFF, and the problem goes away; in that case, NULL = NULL.

Entity Framework Pitfalls: Queries Over Navigation Properties When There Are Foreign Key Properties Are Ignored

Long title, I know! Smile

Another one for Entity Framework Code First. The problem is: imagine you have an entity with a relation to another entity (one-to-one, many-to-one) where you have both the navigation property and the foreign key property:

public class Source
{
public int Id { get; set; }

[ForeignKey("TargetId")]
public virtual Target Target { get; set; }

public int? TargetId { get; set; }
}

public class Target
{
public int Id { get; set; }
public virtual ICollection<Source> Sources { get; set; }
}

You may want to query all Sources that have a Target set:

var sourcesWithTarget = ctx
.Sources
.Where(source => source.Target != null)
.ToList();

However, you will get this SQL instead:

SELECT [Extent1].[Id] AS [Id], [Extent1].[TargetId] AS [TargetId], 
FROM [dbo].[Source] AS [Extent1]

See? No WHERE clause!

If, instead, we query on the foreign key property:

var sourcesWithTarget = ctx
.Sources
.Where(source => source.TargetId != 0)
.ToList();

Then we get what we want:

SELECT [Extent1].[Id] AS [Id], [Extent1].[TargetId] AS [TargetId], 
FROM [dbo].[Source] AS [Extent1]
WHERE 0 <> [Extent1].[TargetId]

Weird, huh?

Stay tuned for more!

Missing Features in Entity Framework Core

Updated: with EF Core 1.1 and 2.0.

Here’s a brief summary of some of the features that were present in previous versions of Entity Framework (6.x) and were excluded (or are not yet implemented) in Entity Framework Core 2.0:

Feature Description Reason/Workaround
Entity Type Configuration The ability to load entity configuration from classes in the same assembly as the context (EntityTypeConfiguration)

https://github.com/aspnet/EntityFramework/issues/2805

Implemented in EF Core 2.0.

Lazy loading The ability to load entity relations after the root entity was loaded, automatically

Will be available in a future version. For now, we need to use eager loading (Include).

https://github.com/aspnet/EntityFramework/issues/3797

Explicit loading The ability to load entity relations after the root entity was loaded, explicitly

https://github.com/aspnet/EntityFramework/issues/3797

Implemented in EF Core 1.1.

Support for Group By Specifying GroupBy in a LINQ query

Currently it falls back silently to LINQ to Objects, meaning, brings everything from the database and groups in memory. Will be available in a future version. For now, use plain SQL.

https://github.com/aspnet/EntityFramework/issues/2341

Support for user defined functions

Using static methods as UDFs

https://github.com/aspnet/EntityFramework/issues/4319

Implemented in EF Core 2.0.

DateTime, TimeSpan operations and common SQL functions Doing DateTime operations and using common SQL functions in LINQ

The ability to do queries involving Date/Time operations (DbFunctions) and common SQL functions (SqlFunctions) in LINQ queries is not supported now. Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/2850

https://github.com/aspnet/EntityFrameworkCore/issues/6025

Complex Values Support for properties of complex types (value objects)

https://github.com/aspnet/EntityFramework/issues/246

Implemented in EF Core 2.0.

Many to Many Collections Many-to-many relations without a middle entity/table

Will be available in a future version. For now, we need a middle entity and table.

https://github.com/aspnet/EntityFramework/issues/1368

Table Splitting

https://github.com/aspnet/EntityFramework/issues/619

Implemented in EF Core 2.0.

Table Per Type Inheritance Strategy

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/2266

Table Per Concrete Type Inheritance Strategy

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/3170

Mapping CUD with stored procedures The ability to use stored procedures for doing inserts, updates and deletes seemlesly

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/245

Map database views The ability to map views instead of tables

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/1679

https://github.com/aspnet/EntityFramework/issues/827

Spatial data types The ability to query and use spatial data types

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/1100

Custom conventions The ability to add custom conventions

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/214

Populate non-model types from SQL Turn the results of custom SQL into classes that are not part of the model

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/240

Connection resiliency support The ability to retry connecting and sending queries

https://github.com/aspnet/EntityFramework/issues/237

Implemented in EF Core 1.1.

Seeding data in migrations The ability to add data when migrating

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/629

Command and query interception The ability to intercept queries and SQL commands

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/626

https://github.com/aspnet/EntityFramework/issues/4048

https://github.com/aspnet/EntityFramework/issues/737

Visual Studio support for generating/updating entities from the database and viewing the model The ability to generate the model from the database from inside Visual Studio and to view the model graphically

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/5837

Database initializers Database initializers Dropped.
Automatic migrations Automatic migrations Dropped.
Pluralization Service Pluralization Service

https://github.com/aspnet/EntityFramework/issues/2506

Implemented in EF Core 2.0.

ObjectContext events SavingChanges and ObjectMaterialized events of ObjectContext

Will be available in a future version.

https://github.com/aspnet/EntityFramework/issues/3204

https://github.com/aspnet/EntityFrameworkCore/issues/626

ObjectContext (Entity SQL) Entity SQL Dropped.
Model first approach Model first approach Dropped.
Data Annotations validations The ability to perform data annotations validations before saving changes Dropped.
Support for System.Transactions The ability to use TransactionScope and distributed transactions

Will be available in a future version.

https://github.com/aspnet/EntityFrameworkCore/issues/5595

Please let me know if you think I missed something! Winking smile

See Microsof’t comparison of Core and pre-Core versions here: https://docs.microsoft.com/en-us/ef/efcore-and-ef6/features.

For the most up to date roadmap, please consult the EF Core roadmap: https://github.com/aspnet/EntityFramework/wiki/Roadmap.

Also, do check out the Entity Framework issue tracker: https://github.com/aspnet/EntityFramework/issues

Entity Framework Pitfalls: Migrations and DbContext Construction

If you want to create or apply a migration over a DbContext-derived context and your context doesn’t have a public parameterless constructor, it will fail. This happens with both .NET Framework and .NET Core: the problem is that the migrations framework has no way of knowing how to create the context so as to get information from it, the migrations framework actually instantiates it!

There are two ways to go around it:

  • Create a public parameterless constructor on your context;
  • Create a class that implements IDbContextFactory<T>, where the generic parameter is your DbContext-derived class.

Having a parameterless constructor is not always ideal, because you may need to pass something in, like a connection string, but you can do it just for executing migrations and them remove it again. Just make sure that the connection string in use is the right one.

As for the IDbContextFactory<T> approach, it’s quite simple:

public class BlogContextFactory : IDbContextFactory<BlogContext>

{

    public BlogContext Create()

    {

        var connectionString = "get a connection string somehow";

        return new BlogContext(connectionString);

    }

}

Basically, you have to implement the Create method so as to return an instance of your context, doesn’t really matter how you instantiate it. The migrations API will find this class automatically.

If you don’t do any of this, you may get weird errors, like, “unable to find the DbContext”, which doesn’t really help much.

Another problem you may face is if you have multiple DbContexts, in that case, you have to tell migrations which one to use.

Entity Framework Pitfalls: Entity Refresh

After an entity is loaded by Entity Framework, it gets stored in its first level cache. After that, whenever EF executes a query that returns this entity, identified by its primary key, EF always returns the cached entry, and makes no attempt to see if the cached data (the rest of the properties) is still up to date with that returned from the query. This feature is common to other ORMs, but it may result in unexpected behavior – we cannot rely on the state of a loaded entity if the database can change by other processes..

The solution for EF pre-Core is either:

  • Remove (evict) the entity from first level cache, by marking it as detached, when we want it to be reloaded from the database:
MyEntity e = ...;

 

ctx.Entry(e).State = EntityState.Detached;

  • Explicitly ask for it to be reloaded:
MyEntity e = ...;

 

ctx.Entry(e).Reload();

In EF Core, there is no Reload method – although I did write an implementation for it –, so the only option is to detach the entity from the context.

Implementing Missing Features in Entity Framework Core – Part 2

This is the second in a series of posts about missing functionality in EF Core. You can find the first here.

Entity Framework used to support three ways to load related entities (one to one, one to many, many to one, many to many):

Entity Framework Core doesn’t include (yet) the explicit loading and lazy loading mechanisms. Lazy loading is not included because EF Core does not generate proxies for the entities it loads, and I guess nobody bothered to implement explicit loading. Well, that’s exactly what we’ll do here!

The idea is, even if we didn’t require a collection to be loaded when we issued the query, we can still load it afterwards. We should keep a syntax similar to the previous version, when this feature was available:

//load a blog

var blog = ctx.Blogs.Single(b => b.BlogId == 1);

 

//Posts collection is empty

blog.Posts.Count(); //0

 

//eager load all Posts

ctx.Entry(blog).Load(b => b.Posts);

 

//Posts collection is no longer empty

blog.Posts.Count(); //!= 0

 

We will create an extension method over EntityEntry<T>:

public static void Load<TSource, TDestination>(this EntityEntry<TSource> entry, Expression<Func<TSource, IEnumerable<TDestination>>> path, Expression<Func<TDestination, TSource>> pathBack = null) where TSource : class where TDestination : class

{

    var entity = entry.Entity;

    var context = entry.Context;

    var entityType = context.Model.FindEntityType(typeof(TSource));

    var keys = entityType.GetKeys();

    var keyValues = context.GetEntityKey(entity);

    var query = context.Set<TDestination>() as IQueryable<TDestination>;

    var parameter = Expression.Parameter(typeof(TDestination), "x");

    PropertyInfo foreignKeyProperty = null;

 

    if (pathBack == null)

    {

        foreignKeyProperty = typeof(TDestination).GetProperties().Single(p => p.PropertyType == typeof(TSource));

    }

    else

    {

        foreignKeyProperty = (pathBack.Body as MemberExpression).Member as PropertyInfo;

    }

 

    var i = 0;

 

    foreach (var property in keys.SelectMany(x => x.Properties))

    {

        var keyValue = keyValues[i];

 

        var expression = Expression.Lambda(

                Expression.Equal(

                    Expression.Property(Expression.Property(parameter, foreignKeyProperty.Name), property.Name),

                    Expression.Constant(keyValue)),

                parameter) as Expression<Func<TDestination, bool>>;

 

        query = query.Where(expression);

 

        i++;

    }

 

    var list = query.ToList();

 

    var prop = (path.Body as MemberExpression).Member as PropertyInfo;

    prop.SetValue(entity, list);

}

The GetEntityKey method was introduced in the previous post, but I will add it here, for your convenience:

public static object[] GetEntityKey<T>(this DbContext context, T entity) where T : class

{

    var state = context.Entry(entity);

    var metadata = state.Metadata;

    var key = metadata.FindPrimaryKey();

    var props = key.Properties.ToArray();

 

    return props.Select(x => x.GetGetter().GetClrValue(entity)).ToArray();

}

You can see that the Load method takes two parameters: the first is required, it represents the collection that we with to load, the second is optional, and it represents the go-back property, in case there is more than one property of the root entity’s type. For example:

ctx.Entry(blog).Load(b => b.Posts, p => p.Blog);

In this case, the path p => p.Blog is needless, because a Post only belongs to one Blog.

A similar approach can be used to load explicitly other kinds of relations (one to one and many to one).

Hope you find this useful! Winking smile

Entity Framework Pitfalls: Attaching New Entities With Existing Related Ones

One of the patterns in EF 6.x for attaching new entities to a context is to simply change it’s state to Added:

ctx.Entry(myEntity).State = EntityState.Added;

Entity Framework 6.x will traverse the entity graph of all the related entities and set all their states to Added too. This might be a problem.

Imagine we want to attach a disconnected (new) entity that is linked to an existing one, for which we already have a reference. In this case, we don’t want this reference to be added, because doing so will raise a violation on the database and it will come out as an exception.

What we need to do is to explicitly set the state of any of these existing instances to Unchanged. Fortunately, Entity Framework Core will solve this with the introduction of a new TrackGraph API.

Thanks for Tony Sneed (@tonysneed) for reminding me.

Entity Framework Pitfalls: Cascade Deletes

Introduction

Entity Framework supports cascading deletes for all of its associations:

  • One to one
  • One to many
  • Many to many

Meaning, when we remove an entity, it will automatically descend to its dependent entities (cascade) and delete them too.

Of course, cascading deletes of many to one really does not make much sense. The problem with cascade deletes configuration in EF is, sometimes it cannot be done with attributes, which is a mapping mechanism that most people like and use, and the different associations need different approaches.

One to One

For one to one relations, we need to use code mappings, say, we have an entity Master associated with a dependent entity Detail:

protected override void OnModelCreating(DbModelBuilder modelBuilder)

{

    modelBuilder

        .Entity<Master>()

        .HasOptional(x => x.Detail)

        .WithRequired(x => x.Master)

        .WillCascadeOnDelete(true);

 

    base.OnModelCreating(modelBuilder);

}

Deleting a Master will also delete the Detail, but, of course, not the other way around. If you are curious, EF does this through an ON DELETE CASCADE INDEX, so it is done by the database, not by EF – more on this later.

image

One to Many

We just need to mark the one endpoint with a [Required] attribute, like in a Detail entity that is a child of a Master:

public class Detail

{

    public int Id { get; set; }

 

    [Required]

    public virtual Master Master { get; set; }

}

This way, when we delete the Master entity, it will first delete all of its related Detail entities. This is also achieved through the automatically created ON DELETE CASCADE INDEX.

It is also certainly possible to achieve this using code configuration, but not really necessary – and more verbose:

protected override void OnModelCreating(DbModelBuilder modelBuilder)

{

    modelBuilder

        .Entity<Master>()

        .HasMany(x => x.Details)

        .WithRequired(x => x.Master)

        .WillCascadeOnDelete();

 

    base.OnModelCreating(modelBuilder);

}

Many to Many

This one is the best: guess what, we don’t have to do anything, it just works out of the box!

Conclusion

Because Entity Framework relies on INDEXes to cascade the deletes, you cannot just change the code mappings after you created the model – either by adding attributes or through fluent configuration – and expect things to work. If you do, you will likely need to update the database. Do so by creating and applying a new migration. One thing that is still (as of EF 6.x) not supported is deleting orphans, but I already talked about it. EF Core will handle it properly.