This seems like a simple problem but I have been having a challenge loading a document via the entity with related document with Marten documentDB on postgres for the past 2 days. I’m also using the latest release of the library (7.26.0). Please patiently go through my work so far:

KycVerificationStep (Entity)

public class KycVerificationStep : BaseEntity
{
    public Guid Id { get; set; }
    public string Code { get; private set; }
    public string Name { get; set; }
    public string Description { get; private set; }
    public string Icon { get; private set; }
    public int Order { get; private set; }
    public DateTimeOffset DateCreated { get; private set; }
    public DateTimeOffset? DateUpdated { get; private set; }

    private KycVerificationStep() { }

    private KycVerificationStep(
        string name,
        string description,
        string icon,
        int order,
        IKycVerificationStepDomainService domainService)
    {
        CheckRule(new KycStepMustBeUniqueRule(domainService, name));
        Name = name;
        Description = description;
        Icon = icon;
        Order = order;
        DateCreated = SystemClock.Now;
        Code = $"Step {Order}";

        AddDomainEvent(new KycVerificationStepCreatedDomainEvent(Id));
    }

    public static KycVerificationStep Create(
        string name,
        string description,
        string icon,
        int order,
        IKycVerificationStepDomainService domainService)
    {
        return new KycVerificationStep(name, description, icon, order, domainService);
    }

    // shortened for brevity   
}

MemberKycVerificationStep

public class MemberKycVerificationStep : BaseEntity
{
    public string Id { get; private set; }

    public Guid KycVerificationStepId { get; private set; }

    public KycVerificationStep KycVerificationStep { get; private set; }

    public Guid MemberId { get; private set; }

    public string Status { get; private set; }

    public DateTimeOffset DateUpdated { get; private set; }

    private MemberKycVerificationStep() { }

    private MemberKycVerificationStep(
        Guid kycVerificationStepId,
        Guid memberId)
    {
        Id = $"{memberId}_{kycVerificationStepId}";
        KycVerificationStepId = kycVerificationStepId;
        MemberId = memberId;
        Status = VerificationStatus.NotStarted.ToString();

        AddDomainEvent(new MemberKycVerificationStepCreatedDomainEvent(MemberId, KycVerificationStepId));
    }

    public static MemberKycVerificationStep Create(
        Guid kycVerificationStepId,
        Guid memberId)
    {
        return new MemberKycVerificationStep(kycVerificationStepId, memberId);
    }
}

Here’s how I try to query MemberKycVerificationStep the get the KycVerificationStep By MemberId.

public async Task<Result<IPagedList<MemberKycVerificationStepDto>>> Handle(GetAllVerificationStepsByUserQuery request, CancellationToken cancellationToken)
{
    KycVerificationStep included = null;
    var results = await _querySession
    .Query<MemberKycVerificationStep>()
        .Include<KycVerificationStep>(x => included = x)
            .On(x => x.KycVerificationStepId)
        .Where(x => x.MemberId == request.UserId)
        .Select(x => new MemberKycVerificationStepDto
        {
            KycVerificationStepId = x.KycVerificationStepId,
            Name = included != null ? included.Name : "",
            DateUpdated = x.DateUpdated,
            Status = "x.Status",
            MemberId = x.MemberId
        })
        .ToPagedListAsync(request.PageIndex, request.PageSize, cancellationToken);
    return await Result<IPagedList<MemberKycVerificationStepDto>>.SuccessAsync(results);
}

But I get the following exception which comes from the linq expressions implementation from Marten.

System.ArgumentNullException: Value cannot be null. (Parameter ‘key’)
at System.Collections.Generic.Dictionary2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) at System.Collections.Generic.Dictionary2.set_Item(TKey key, TValue
value) at
Marten.Linq.Parsing.SelectParser.VisitConstant(ConstantExpression
node) at Marten.Linq.Parsing.SelectParser.Visit(Expression node)
at
System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression
node) at
Marten.Linq.Parsing.SelectParser.VisitBinary(BinaryExpression node)
at Marten.Linq.Parsing.SelectParser.Visit(Expression node) at
System.Linq.Expressions.ExpressionVisitor.VisitConditional(ConditionalExpression
node) at Marten.Linq.Parsing.SelectParser.Visit(Expression node)
at Marten.Linq.Parsing.SelectParser.ReadBinding(MemberAssignment
binding) at
Marten.Linq.Parsing.SelectParser.VisitMemberInit(MemberInitExpression
node) at Marten.Linq.Parsing.SelectParser.Visit(Expression node)
at Marten.Linq.Parsing.SelectParser..ctor(ISerializer serializer,
IQueryableMemberCollection members, Expression expression) at
Marten.Linq.Parsing.SelectorVisitor.ToSelectTransform(Expression
selectExpression, ISerializer serializer) at
Marten.Linq.Parsing.SelectorVisitor.VisitMemberInit(MemberInitExpression
node) at Marten.Linq.Parsing.SelectorVisitor.Visit(Expression node)
at Marten.Linq.CollectionUsage.BuildTopStatement(IMartenSession
session, IQueryableMemberCollection collection, IDocumentStorage
storage, QueryStatistics statistics) at
Marten.Linq.Parsing.LinqQueryParser.BuildStatements() at
Marten.Linq.Parsing.LinqQueryParser.BuildListHandlerT at
Marten.Linq.MartenLinqQueryable1.ToListAsync[TResult](CancellationToken token) at Marten.Pagination.PagedList1.InitAsync(IQueryable1 queryable, Int32 pageNumber, Int32 pageSize, CancellationToken token) at Marten.Pagination.PagedList1.CreateAsync(IQueryable1 queryable, Int32 pageNumber, Int32 pageSize, CancellationToken token) at Marten.Pagination.PagedListQueryableExtensions.ToPagedListAsync[T](IQueryable1
queryable, Int32 pageNumber, Int32 pageSize, CancellationToken token)

The data for the MemberKycVerificationStep document is present in the db. But I don’t understand why I’m getting a null reference. Please help.