Dasync/EntityFrameworkCore.Extensions.Projections

Extension for EFCore that decouples the read model from entities themselves

A small weekend project with a higher future purpose

This extension to the EntityFramework Core allows to decouple the read model from entities themselves. If you practice Domain-Driven Design, then you want to separate Domain Entity/Aggregate repositories from their querying mechanism. Query results must return Entity Projections/Views instead of Entities themselves (as they contain behavior), but building projection types by hand is tedious.

The full description is published in the Domain Entity Projections in EFCore post.

Example

This is a Domain Entity that contains behaviors and implements a projection interface (as a contract) defined below.

public class City : ICityProjection
{
    public string Name { get; private set; }

    public string State { get; private set; }

    public long Population { get; private set; }

    public int TimeZone { get; private set; }

    public void SwitchToSummerTime()
    {
        TimeZone += 1;
    }
}

A sample projection interface. An entity can have implement multiple projection interfaces.

public interface ICityProjection
{
    string Name { get; }

    string State { get; }

    long Population { get; }
}

Just use the HasProjections extension method on the desired entity(-ies). That will automatically find all interfaces with get-only properties.

using Dasync.EntityFrameworkCore.Extensions.Projections;

public class SampleDbContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<City>(e =>
        {
            // Declare that this entity has projection interfaces.
            e.HasProjections();
        });
    }
}

Time to query entities using their projection.

var smallCities = await dbContext
    .Set<ICityProjection>() // Query directly on the projection interface
    .Where(c => c.Population < 1_000_000)
    .ToListAsync();

Note that the result set does not contain instances of the original City entity type. Instead, this extension library generates types at runtime that implement given projection interfaces.

Then you can safely serialize your result set as it represents projections but not entities with behavior. Useful for API methods.

var json = JsonConvert.SerializeObject(smallCities);

Deserialization back can be done without involving entities as well (visit the 'samples' folder to see how EntityProjectionJsonConverter is implemented).

var cityProjections = JsonConvert.DeserializeObject<List<ICityProjection>>(
    json, EntityProjectionJsonConverter.Instance);

How to start

See the 'samples' folder, but in a nutshell:

1. Add Dasync.EntityFrameworkCore.Extensions.Projections NuGet Package to your app
2. Define projection interfaces and implement them on your entities
3. Add using Dasync.EntityFrameworkCore.Extensions.Projections; to your code
4. Use HasProjections extension method on entities while building your DbContext model
5. Query with projection interfaces as shown above
6. Serialize projections at the API layer