c# - Either get local entity or attach a new one

Question

My Entity looks like this:

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

I don't want to query the database each time I have get a specific User where I know a User exists for this Id. Seems like Attach works for this case but If the DbContext already stores the Entity for this specific User locally It will throw an exception.

Example what I want to do:

var user1 = ctx.GetLocalOrAttach(new User{Id = 1});
var user2 = ctx.GetLocalOrAttach(new User{Id = 2});
AddUserRelation(user1, user2);

Is there some solution for this? If not what would be the ideal way to check if an Entity exists locally.

Answer 1

You can search the DbSet<T>.Local property, but that would be inefficient.

A better way IMO is to use the FindTracked custom extension method from my answer to Delete loaded and unloaded objects by ID in EntityFrameworkCore

using Microsoft.EntityFrameworkCore.Internal;

namespace Microsoft.EntityFrameworkCore
{
    public static partial class CustomExtensions
    {
        public static TEntity FindTracked<TEntity>(this DbContext context, params object[] keyValues)
            where TEntity : class
        {
            var entityType = context.Model.FindEntityType(typeof(TEntity));
            var key = entityType.FindPrimaryKey();
            var stateManager = context.GetDependencies().StateManager;
            var entry = stateManager.TryGetEntry(key, keyValues);
            return entry?.Entity as TEntity;
        }
    }
}

which is similar to EF Core Find method, but does not load the entity from the database if it doesn't exist locally.

The usage with your case would be like this:

var user1 = ctx.FindTracked(1) ?? ctx.Attach(new User { Id = 1 }).Entity;
var user2 = ctx.FindTracked(2) ?? ctx.Attach(new User { Id = 2 }).Entity;
AddUserRelation(user1, user2);