Data access

Arc4u does not replace Entity Framework Core, the MongoDB driver or ASP.NET Core OData. It adds a small layer on top of them for the data access layer of a service: include paths (Graph<T>) applied to Entity Framework Core queries, a DbContext-like abstraction for MongoDB, and the two settings OData needs behind a reverse proxy.

What it solves

A service persists the aggregates of its domain model. The domain model decides what changes (its methods add, edit or remove children); the data access layer only loads and saves aggregates with the underlying library. Arc4u does not track changes on entities: use the change tracking of Entity Framework Core, or the update API of the MongoDB driver.

flowchart LR
    Domain[Domain model] --> DAL[Data access layer]
    DAL -->|Graph&lt;T&gt; includes| EF[Entity Framework Core]
    DAL -->|IMongoClientFactory| Mongo[MongoDB driver]

Arc4u deliberately leaves the following to the underlying libraries: choosing an EF Core provider, mapping, migrations, querying, and everything about the MongoDB driver except the client and collection lookup. Arc4u.OData is not an OData implementation: it only fixes the URLs in OData responses when a service sits behind a proxy.

Packages involved

Package	Use it for
`Arc4u.EfCore`	Applying `Graph<T>` includes to queries. See Entity Framework Core.
`Arc4u.MongoDB`	Registering a MongoDB database context and getting collections mapped to entity types. See MongoDB.
`Arc4u.OData`	Rewriting the URLs in OData responses behind a reverse proxy. See OData.

The packages do not reference each other. Graph<T> lives in the base Arc4u package, which Arc4u.EfCore references, so a caller can build a graph without referencing EF Core. See Include graphs.

Install

Install only what the service uses:

dotnet add package Arc4u.EfCore --prerelease
dotnet add package Arc4u.MongoDB --prerelease
dotnet add package Arc4u.OData --prerelease

You also need an EF Core provider package (for example Microsoft.EntityFrameworkCore.SqlServer) or a reachable MongoDB server; Arc4u ships neither.

Configuration

Arc4u.EfCore and Arc4u.OData read no configuration section. Arc4u.MongoDB reads one connection string from the standard ConnectionStrings section, whose name you pass to AddMongoDatabase. The keys are described in MongoDB.

Common scenarios

Load an aggregate with its children from EF Core

using Arc4u;
using Arc4u.EfCore;
using Microsoft.EntityFrameworkCore;

public class OrderRepository(ShopContext db)
{
    public Task<List<Order>> GetAllAsync(Graph<Order> graph, CancellationToken cancellationToken) =>
        graph.ApplySetReferences(db.Orders).ToListAsync(cancellationToken);
}

ShopContext and Order are defined in Entity Framework Core.

Read a MongoDB collection mapped to an entity

using Arc4u.MongoDB;
using MongoDB.Driver;

public class ProductStore(IMongoClientFactory<ShopContext> factory)
{
    public Task<List<Product>> GetAllAsync(CancellationToken cancellationToken) =>
        factory.GetCollection<Product>().Find(FilterDefinition<Product>.Empty).ToListAsync(cancellationToken);
}

ShopContext and Product are defined in MongoDB.

Expose OData behind a gateway

See OData.

Extensibility points

Service	Default implementation	Replace it to
`IMongoClientFactory<TContext>`	DefaultMongoClientFactory<TContext>	Control how the client and database are created. `AddMongoDatabase` registers the default with `TryAdd`, so register your own before calling it.

Arc4u.EfCore is a static helper, without dependency injection registrations.

Troubleshooting

`MongoClientException: No mongo client settings defined for key ...`

DefaultMongoClientFactory<TContext>.CreateClient found no MongoClientSettings registered for the lower-case database name of the context. AddMongoDatabase registers them under that name, so this only happens when you register the context or the settings yourself. See MongoDB troubleshooting.

More symptoms

Symptoms specific to one package are on its page: Entity Framework Core, MongoDB, OData.

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