Graph Reconciler for Entity Framework 6 and Core

Warning: The EF Core variant in versions prior to 0.3 on .NET Core became buggy with a change in semantics of later EF Core releases. I don't know with exactly which release of EF Core this issue manifested, but under certain circumstances you would delete more entities than you asked for. The test suite shows this and since 0.3 this is fixed. I recommend not using the earlier versions unless you're sure your version of EF Core passes the test suite - I only know for sure that EF Core 2.1 does.

Teaser

This library allows you to write

    await context.ReconcileAsync(personSentFromClient, e => e
        .WithOne(p => p.Address)
        .WithMany(p => p.Tags, e2 => e2
            .WithShared(p => p.Tag))
        ;
    await context.SaveChangesAsync();

and it will sync the four respective tables to match the given, detached personSentFromClient entity by adding, updating and removing entities as required.

Its primary use case are updates on multiple related entities retrieved from a client through an API.

It is a replacement for the GraphDiff library.

The EF 6 and EF Core versions share the same source code as far as possible to ensure consistency.

NuGet

There is one NuGet package for each of the two frameworks:

Install-Package Reconciler.Ef6

Install-Package Reconciler.EfCore

The EF6 version also has a prerelease package that targets netstandard2.1 and EF6.3 for use with .NET Core.

Definitions

• Template entities are the entities to reconcile towards (personSentFromClient in the teaser sample)
• The extent is the extent of the subtree rooted in the template entity of the first parameter that is to be reconciled as defined by the second parameter to the Reconcile extension methods.

Further features

The extent definition can also contain certain extra information to help with common scenarios that would otherwise often require some convoluted manual code.

Fixing

Sometimes we need to employ certain fixes on nested parts of the graph on saving:

.OnInsertion(e => e.Id == Guid.NewGuid())
.OnInsertion(e => e.CreatedAt == DateTimeOffset.Now)
.OnUpdate(e => e.ModifiedAt == DateTimeOffset.Now)

The OnUpdate definitions does not also apply (as of version 1.2.0) to insertions.

Note the use of the equality operator, as the assignment operator can't be used in expression trees in C#.

Exclude properties

Sometimes some properties should not be updated, and sometimes they shouldn't even be passed to a client on loading:

.WithReadOnly(e => e.Unmodifiable)
.WithBlacked(e => e.Secret)

The latter implies .WithReadOnly on writing since version 1.2.0 so that saving what was previously loaded doesn't accidentally overwrite the blacked field.

Some details and caveats

The are some things to be aware of:

• I'm using the library in production, but that doesn't mean it's mature. The test suite is thin and you may hit issues.
• Specifying relationships on derived classes in models with inheritance is not supported.
• Using entities that are part of an inheritance hierarchy in all other contexts is also untested and likely doesn't work yet.
• Many-to-many relationships are not supported and will probably never be.
• Key Consistency Requirement: In the EF6 version, the foreign key properties in the template entity must be set to match the respective navigational properties before the call to one of the Reconcile overloads is made. For example, it should be that person.AddressId == person.Address.Id in the unit test's sample model. For EF Core, this requirement was lifted in version 1.1.0.
• The extent must represent a subtree, i.e. have no cycles, and all entities must appear only once.
• The Reconcile overloads themselves access the database only for reading and thus need to be followed by a SaveChanges call. Alternatively, ReconcileAndSaveChanges does that for you.
• A Reconcile call should normally be followed by a SaveChanges. Multiple Reconcile calls without saving in between will likely only work properly if the datasets involved are disjoint.
• The number of reads done is the number of entities either in storage or in the template that are covered by the extent and have a non-trivial sub-extent themselves. In the above example, the address would have been fetched with the person itself and cause no further load, but there would be one load per tag-to-person bridge table row which each include the respective tag. There's room for optimization here, but that's where it's at.

Some things didn't work in the past but are supported since version 1.0.0:

• Database-generated keys such as auto-increment integers should now be well-behaved.
• Moving entities from one collection to another will work if those collections are on the same extent level and the move is done in a single Reconcile call: Consider for example the model Stars > Planets > Moons with three entity types, two of which have a foreign key to the object they are orbiting. You can move a moon to a different planet while reconciling on a star. However, you need the extra level of the star so that you can express this operation in a single call to Reconcile (as you need at least two planets for a move of a moon to occur).

GraphDiff

Before writing this replacement I used the GraphDiff library, but since it is no longer maintained I became motivated to write my own solution. In particular, I wanted

• Entity Framework Core support,
• async/await support and
• really understand what's going on.

As I don't fully understand the original there are some differences beyond the names of functions: GraphDiff had OwnedEntity, OwnedCollection and AssociatedCollection as options for how to define the extent, and I don't quite know what the difference between associated and owned is.

Reconciler has WithOne, WithMany and WithShared:

WithOne and WithMany reconcile a scalar or collection navigational property, respectively, through respective addition, update and removal operations. WithShared only works on scalar navigational properties and doesn't remove a formerly related entity that is now no longer related. This is useful, for example, in the sample in the teaser where we want to insert new tag entities when they are needed but don't want to remove them as they may be shared by other entities.

Roadmap

There are a number of exciting features that would make this library much more useful. See this document for further information.