Is there an option to extend the model without inheriting from DbContext?

[voroninp]

For example, I am developing a NuGet library which should support various DB libraries including EF.

Is there any option to extend data model other than requiring DbContext to implement a particular interface or inherit from another context?

[roji]

Your nuget package can integrate into the model-building process by introducing its own conventions; as an example, see EFCore.CheckConstraints which adds various check constraints to the model.

To help out more, we'd need to know exactly what kind of model changes your library intends to do etc.

[voroninp]

Very often, I need to extend entities with service properties like Timestamp or Version and manage such properties automatically.

For example, Version can be a (shadow) property which CurrentValue is incremented right before the changes are saved to DB.

[voroninp]

Or I just need to add some table for library-specific data. Like Outbox for published messages.

[roji]

Then yeah, plugin conventions seem like the thing to do - take a look at EFCore.CheckConstraints.

[voroninp]

Ok, thanks, I'll look at it.

And if I want to intercept expression tree manipulation?

As I said before, I'd like to express domain behavior in methods which work exclusively with entity's state. EF cannot translate these methods to SQL, but if method call could be replaced with its body, EF will do fine.

So I want an extension coupled with source generator.

public sealed class Foo
{
    public int Prop1 {get; set;}
    public string Prop2 {get; set;}

    public bool IsBar() => Prop1 == 0 && Prop2 == "bar";
}

If I call

ctx.Set<Foo>().Where(f => f.IsBar()).ToList()`

EF will understandably throw.

So, my idea is to have a source generator which checks for instance method calls and creates expression equivalents:

Expression<Func<Foo, bool>> isBarExpr = foo => Prop1 == 0 && Prop2 == "bar";

The second step would be to replace Where(() => f.IsBar()) with Where(isBarExpr).

It can be done without source generator if explicit mapping between the method and expression is provided.

[roji]

A plugin can also add method and member translators - you can look at the NodaTime and/or NetTopologySuite plugins in this repo for inspiration.

I'll go ahead and close this now as the questions have been answered, but feel free to post back here if you need to.

[voroninp]

@roji Q1: Do I get it right that annotations is the way to extend configuration of entites?

For example, I want to mark a property as a timestamp.

b.Property(e => e.TimestampUtc).IsTimestamp();

public static PropertyBulder<DateTime> IsTimestamp(this PropertyBulder<DateTime> propertyBuilder, bool isTimeStamp = true)
{
    return propertyBuilder.HasAnnotation("voroninp:IsTimeStamp", isTimeStamp);
}

[roji]

@voroninp in general yes, but I'd need to know more about exactly what you're trying to achieve.

[voroninp]

@roji

1. Mark a property to work as a timestamp. Either with attribute or with fluent mapping.
2. When DbContext performs SaveChanges, scan through the entities (Added/Modified), find those with timestamp properties and assign new value to the property.
3. The value should be acquired via TimeProvider which is injected to the service which does the job of updating the property.

[roji]

Sounds like you want to set a value generator that works on update (not yet supported, #6999). Until then, although annotations are meant more as an internal and provider thing, you may be able to use that as a workaround, yes. Another option is to get the timestamp generated in the databases e.g. via a trigger (doc). /cc @ajcvickers in case he has other ideas.

[voroninp]

Well, currently I can just override SaveChanges and do the work there. But I'd like to have it as an extension. Currently not possible?

[voroninp]

Also, how can I add annotation to the entity via PropertyBuilder? For example, I want entity to be annotated with voroininp:HasTimestampProperty when I call .IsTimestamp().

[roji]

Not sure what you're asking - you seem to already be using PropertyBuilder.HasAnnotation() - IsTimestamp() can be an extension over PropertyBuilder which just does that, no?

[voroninp]

I mean, I add annotation to the property, but I'd like to add it to the entity as well when I call IsTimestamp()

[roji]

You can drop down to the lower-level metadata API and get the property's declaring entity type:

modelBuilder.Entity<Blog>().Property(f => f.Id).Metadata.DeclaringType.SetAnnotation();

[voroninp]

Ok, thanks.
Next question.
Which service do I need to extend/replace to intercept saving changes?

[voroninp]

ping...