Read & Property Filters

Filters let you enforce cross-cutting rules on a repository without repeating them at every call site. They are registered on the repository interface via attributes and resolved from the dependency injection container, so they can depend on other services (for example the current user or tenant).

There are two kinds of filters:

• Read filters restrict which entities are visible.
• Property filters modify which data of the returned entities is exposed.

Read filters

A read filter contributes a predicate that is combined (logical AND) with every read operation. Entities that do not match are invisible to the repository: they are excluded from QueryAsync, GetAllAsync, IterateAsync and CountAsync, and GetAsync throws a NotFoundErrorException for them.

Implement IDatabaseRepositoryReadFilter<TEntity>:

GeneralUserReadFilter.cs

using System;
using System.Linq.Expressions;
using System.Threading.Tasks;
using Wemogy.Infrastructure.Database.Core.Abstractions;

public class GeneralUserReadFilter : IDatabaseRepositoryReadFilter<User>
{
    public Task<Expression<Func<User, bool>>> FilterAsync()
    {
        return Task.FromResult((Expression<Func<User, bool>>)(x => x.Firstname != "John"));
    }
}

Register it with the RepositoryReadFilter attribute (multiple filters are allowed and all of them must match):

IUserRepository.cs

[RepositoryReadFilter(typeof(GeneralUserReadFilter))]
public interface IUserRepository : IDatabaseRepository<User>
{
}

Use case

Read filters are a robust way to implement row-level security (for example "only return entities owned by the current user"), because the rule is applied centrally and cannot be forgotten at a call site.

Property filters

A property filter runs over the entities returned by a read operation and can mutate them, for example to strip sensitive fields. It does not change what is stored in the database, only what the caller receives.

Implement IDatabaseRepositoryPropertyFilter<TEntity>:

GeneralUserPropertyFilter.cs

using System.Collections.Generic;
using System.Threading.Tasks;
using Wemogy.Infrastructure.Database.Core.Abstractions;

public class GeneralUserPropertyFilter : IDatabaseRepositoryPropertyFilter<User>
{
    public Task FilterAsync(List<User> entities)
    {
        foreach (var entity in entities)
        {
            entity.PrivateNote = string.Empty;
        }

        return Task.CompletedTask;
    }
}

Register it with the RepositoryPropertyFilter attribute (multiple filters are allowed):

IUserRepository.cs

[RepositoryPropertyFilter(typeof(GeneralUserPropertyFilter))]
public interface IUserRepository : IDatabaseRepository<User>
{
}

The filter is applied to all read operations, so the PrivateNote of every user returned by GetAsync, QueryAsync, GetAllAsync or IterateAsync is cleared.