Extending Ardalis.Specification for NHibernate with Linq, Criteria API, and Query Over

Ardalis.Specification is a powerful library that enables
the specification pattern for querying databases, primarily designed for Entity Framework Core,
but here I'll demonstrate how you can extend Ardalis.Specification to use NHibernate as an ORM as well.

This blog post assumes you have some experience with Ardalis.Specification, and want to use it in a
project using NHibernate. If you are not familiar with Ardalis.Specification yet, head over to the
documentation to learn more.

First, in NHibernate there are three different built-in ways to perform queries

• Linq to query (using IQueryable)
• Criteria API
• Query Over

I'll go through how you can extend Ardalis.Specification to handle all 3 ways, but since Linq to Query
also works with IQueryable like Entity Framework Core, I'll go through that option first.

Linq to query

There is a small nuance between Entity Framework Core and NHIbernate when it comes to create join relationships.
In Entity Framework Core we have extensions methods on IQueryable: Include and ThenInclude
(these are also the method names used in Ardalis.Specification).
Fetch, FetchMany, ThenFetch and ThenFetchMany are NHibernate specific methods on IQueryable that do joins.
The IEvaluator gives us the extensibility we need to invoke the correct extension method when we work with NHibernate.

Add an implementation of IEvaluator as follows:

public class FetchEvaluator : IEvaluator
{
   private static readonly MethodInfo FetchMethodInfo = typeof(EagerFetchingExtensionMethods)
        .GetTypeInfo().GetDeclaredMethods(nameof(EagerFetchingExtensionMethods.Fetch))
        .Single();

   private static readonly MethodInfo FetchManyMethodInfo = typeof(EagerFetchingExtensionMethods)
       .GetTypeInfo().GetDeclaredMethods(nameof(EagerFetchingExtensionMethods.FetchMany))
       .Single();

   private static readonly MethodInfo ThenFetchMethodInfo
       = typeof(EagerFetchingExtensionMethods)
           .GetTypeInfo().GetDeclaredMethods(nameof(EagerFetchingExtensionMethods.ThenFetch))
           .Single();

   private static readonly MethodInfo ThenFetchManyMethodInfo
       = typeof(EagerFetchingExtensionMethods)
           .GetTypeInfo().GetDeclaredMethods(nameof(EagerFetchingExtensionMethods.ThenFetchMany))
           .Single();

    public static FetchEvaluator Instance { get; } = new FetchEvaluator();

    public IQueryable<T> GetQuery<T>(IQueryable<T> query, ISpecification<T> specification) where T : class
    {
        foreach (var includeInfo in specification.IncludeExpressions)
        {
            query = includeInfo.Type switch
            {
                IncludeTypeEnum.Include => BuildInclude<T>(query, includeInfo),
                IncludeTypeEnum.ThenInclude => BuildThenInclude<T>(query, includeInfo),
                _ => query
            };
        }

        return query;
    }

    public bool IsCriteriaEvaluator { get; } = false;

    private IQueryable<T> BuildInclude<T>(IQueryable query, IncludeExpressionInfo includeInfo)
    {
        _ = includeInfo ?? throw new ArgumentNullException(nameof(includeInfo));

        var methodInfo = (IsGenericEnumerable(includeInfo.PropertyType, out var propertyType)
            ? FetchManyMethodInfo 
            : FetchMethodInfo);

       var method = methodInfo.MakeGenericMethod(includeInfo.EntityType, propertyType);

       var result = method.Invoke(null, new object[] { query, includeInfo.LambdaExpression });
        _ = result ?? throw new TargetException();

        return (IQueryable<T>)result;
    }

    private IQueryable<T> BuildThenInclude<T>(IQueryable query, IncludeExpressionInfo includeInfo)
    {
        _ = includeInfo ?? throw new ArgumentNullException(nameof(includeInfo));
        _ = includeInfo.PreviousPropertyType ?? throw new ArgumentNullException(nameof(includeInfo.PreviousPropertyType));

        var method = (IsGenericEnumerable(includeInfo.PreviousPropertyType, out var previousPropertyType)
            ? ThenFetchManyMethodInfo
            : ThenFetchMethodInfo);

        IsGenericEnumerable(includeInfo.PropertyType, out var propertyType);

        var result = method.MakeGenericMethod(includeInfo.EntityType, previousPropertyType, propertyType)
            .Invoke(null, new object[] { query, includeInfo.LambdaExpression });

        _ = result ?? throw new TargetException();

        return (IQueryable<T>)result;
    }

    private static bool IsGenericEnumerable(Type type, out Type propertyType)
    {
        if (type.IsGenericType && (type.GetGenericTypeDefinition() == typeof(IEnumerable<>)))
        {
            propertyType = type.GenericTypeArguments[0];

            return true;
        }

        propertyType = type;

        return false;
    }
}

Next we need to configure ISpecificationEvaluator to use our FetchEvaluator (and other evaluators).
We add an implementation ISpecificationEvaluator as follows with the Evaluators configured in the constructor.
WhereEvaluator, OrderEvaluator and PaginationEvaluator are all in the Ardalis.Specification
and works well NHibernate as well.

public class LinqToQuerySpecificationEvaluator : ISpecificationEvaluator
{
    private List<IEvaluator> Evaluators { get; } = new List<IEvaluator>();

    public LinqToQuerySpecificationEvaluator()
    {
        Evaluators.AddRange(new IEvaluator[]
        {
            WhereEvaluator.Instance,
            OrderEvaluator.Instance,
            PaginationEvaluator.Instance,
            FetchEvaluator.Instance
        });
    }


    public IQueryable<TResult> GetQuery<T, TResult>(IQueryable<T> query, ISpecification<T, TResult> specification) where T : class
    {
        if (specification is null) throw new ArgumentNullException(nameof(specification));
        if (specification.Selector is null && specification.SelectorMany is null) throw new SelectorNotFoundException();
        if (specification.Selector is not null && specification.SelectorMany is not null) throw new ConcurrentSelectorsException();

        query = GetQuery(query, (ISpecification<T>)specification);

        return specification.Selector is not null
            ? query.Select(specification.Selector)
            : query.SelectMany(specification.SelectorMany!);
    }

    public IQueryable<T> GetQuery<T>(IQueryable<T> query, ISpecification<T> specification, bool evaluateCriteriaOnly = false) where T : class
    {
        if (specification is null) throw new ArgumentNullException(nameof(specification));

        var evaluators = evaluateCriteriaOnly ? Evaluators.Where(x => x.IsCriteriaEvaluator) : Evaluators;

        foreach (var evaluator in evaluators)
           query = evaluator.GetQuery(query, specification);

        return query;
    }
}

Now we can create a reference to LinqToQuerySpecificationEvaluator
in our repository that may look something like this:

public class Repository : IRepository
{
    private readonly ISession _session;
    private readonly ISpecificationEvaluator _specificationEvaluator;

    public Repository(ISession session)
    {
        _session = session;
        _specificationEvaluator = new LinqToQuerySpecificationEvaluator();
    } 

    ... other repository methods

    public IEnumerable<T> List<T>(ISpecification<T> specification) where T : class
    {
        return _specificationEvaluator.GetQuery(_session.Query<T>().AsQueryable(), specification).ToList();
    }

    public IEnumerable<TResult> List<T, TResult>(ISpecification<T, TResult> specification) where T : class
    {    
        return _specificationEvaluator.GetQuery(_session.Query<T>().AsQueryable(), specification).ToList();
    }

    public void Dispose()
    {
        _session.Dispose();
    }
}

That's it. We can now use Linq to Query in our specifications just like we normally do with Ardalis.Specification:

public class TrackByName : Specification<Core.Entitites.Track>
{
    public TrackByName(string trackName)
    {
        Query.Where(x => x.Name == trackName);
    }
}

Now that we've covered Linq-based queries, let's move on to handling Criteria API and Query Over, which require a different approach.

Mixing Linq, Criteria, and Query Over in NHibernate

Since Criteria API and Query Over has their own implementation to generate SQL, and doesn't use IQueryable,
they are incompatible with the IEvaluator interface. My solution is to avoid using the IEvaluator interface for these methods
in this case, but rather focus on the benefits of the specification pattern. But I also want to be able to mix
Linq to Query, Criteria and Query Over with in my solution (if you only need one of these implementations, you can
cherry-pick for your best needs).

To be able to do that, I add four new classes that inherits Specification or Specification

NOTE: The assembly where you define these classes needs a reference to NHibernate as we define actions
for Criteria and a QueryOver, that can be found in NHibernate

public class CriteriaSpecification<T> : Specification<T>
{
    private Action<ICriteria>? _action;
    public Action<ICriteria> GetCriteria() => _action ?? throw new NotSupportedException("The criteria has not been specified. Please use UseCriteria() to define the criteria.");
    protected void UseCriteria(Action<ICriteria> action) => _action = action;
}

public class CriteriaSpecification<T, TResult> : Specification<T, TResult>
{
    private Action<ICriteria>? _action;
    public Action<ICriteria> GetCriteria() => _action ?? throw new NotSupportedException("The criteria has not been specified. Please use UseCriteria() to define the criteria.");
    protected void UseCriteria(Action<ICriteria> action) => _action = action;
}

public class QueryOverSpecification<T> : Specification<T>
{
    private Action<IQueryOver<T, T>>? _action;
    public Action<IQueryOver<T, T>> GetQueryOver() => _action ?? throw new NotSupportedException("The Query over has not been specified. Please use the UseQueryOver() to define the query over.");
    protected void UseQueryOver(Action<IQueryOver<T, T>> action) => _action = action;
}

public class QueryOverSpecification<T, TResult> : Specification<T, TResult>
{
    private Func<IQueryOver<T, T>, IQueryOver<T, T>>? _action;
    public Func<IQueryOver<T, T>, IQueryOver<T, T>> GetQueryOver() => _action ??  throw new NotSupportedException("The Query over has not been specified. Please use the UseQueryOver() to define the query over.");
    protected void UseQueryOver(Func<IQueryOver<T, T>, IQueryOver<T, T>> action) => _action = action;
}

Then we can use pattern matching in our repository to change how we do queries with NHibernate

public IEnumerable<T> List<T>(ISpecification<T> specification) where T : class
{
    return specification switch
    {
        CriteriaSpecification<T> criteriaSpecification => 
            _session.CreateCriteria<T>()
                .Apply(query => criteriaSpecification.GetCriteria().Invoke(query))
                .List<T>(),

        QueryOverSpecification<T> queryOverSpecification => 
            _session.QueryOver<T>()
                .Apply(queryOver => queryOverSpecification.GetQueryOver().Invoke(queryOver))
                .List<T>(),

        _ => _specificationEvaluator.GetQuery(_session.Query<T>().AsQueryable(), specification).ToList()
    };
}

public IEnumerable<TResult> List<T, TResult>(ISpecification<T, TResult> specification) where T : class
{

    return specification switch
    {
        CriteriaSpecification<T, TResult> criteriaSpecification => 
            _session.CreateCriteria<T>()
                .Apply(query => criteriaSpecification.GetCriteria().Invoke(query))
                .List<TResult>(),

        QueryOverSpecification<T, TResult> queryOverSpecification =>
            _session.QueryOver<T>()
                .Apply(queryOver => queryOverSpecification.GetQueryOver().Invoke(queryOver))
                .List<TResult>(),

        _ => _specificationEvaluator.GetQuery(_session.Query<T>().AsQueryable(), specification).ToList()
    };
}

The Apply() method above are an extension method that simplifies the query to a single line:

public static class QueryExtensions
{
    public static T Apply<T>(this T obj, Action<T> action)
    {
        action(obj);
        return obj;
    }

    public static TResult Apply<T, TResult>(this T obj, Func<T, TResult> func)
    {
        return func(obj);
    }
}

Criteria specification example

NOTE: The assembly where you define these classes needs a reference to NHibernate as we define actions
for Criteria, that can be found in NHibernate

public class TrackByNameCriteria : CriteriaSpecification<Track>
{
    public TrackByNameCriteria(string trackName)
    {
        this.UseCriteria(criteria => criteria.Add(Restrictions.Eq(nameof(Track.Name), trackName)));
    }
}

Query over specification example

NOTE: The assembly where you define these classes needs a reference to NHibernate as we define actions
for a QueryOver, that can be found in NHibernate

public class TrackByNameQueryOver : QueryOverSpecification<Track>
{
    public TrackByNameQueryOver(string trackName)
    {
        this.UseQueryOver(queryOver => queryOver.Where(x => x.Name == trackName));
    }
}

By extending Ardalis.Specification for NHibernate, we unlock the ability to use Linq to Query,
Criteria API, and Query Over—all within a single repository pattern. This approach offers a
highly adaptable and powerful solution for NHibernate users