How to register own Mapper?

[thedomeffm]

We want to use Guid as our primary key.

This fails because of: MappingFailedException Cannot map record to type Skill because the record does not contain a value for the property 'nodeId'.

What are we doing wrong?

using Neo4j.Driver.Mapping;

// ...

public class Skill
{
    [MappingSource("nodeId")]
    public Guid nodeId { get; set; }
    
    [Required]
    [MappingSource("name")]
    public string name { get; set; };
}

public class SkillRepository {
    // ...

    public async Task<Skill> Upsert(Skill skill) { /* ...  */ }

    public async Task<Skill> GetById(Guid nodeId)
    {
        StringBuilder queryBuilder = new StringBuilder("MATCH (skill:Skill {nodeId: $nodeId}) RETURN skill");
    
        IList<IRecord> result = await _neo4JDriverService.ExecuteReadQueryAsync(queryBuilder.ToString(), new { nodeId = nodeId.ToString() });
    
        RecordObjectMapping.Register(new GuidMapper()); // <-- Is this correct?

        RecordObjectMapping.RegisterProvider(new GuidMappingProvider()); // or this..?
    
        return result.Single().AsObject<Skill>();
    }
}

public class GuidMapper : IRecordMapper<Guid>
{
    public Guid Map(IRecord record)
    {
        // does not work:
        return Guid.Parse(record.ToString());
    }
}

public class GuidMappingProvider : IMappingProvider
{
    public void CreateMappers(IMappingRegistry registry)
    {
        registry
            .RegisterMapping<Skill>(
                builder =>
                {
                    builder
                        .UseDefaultMapping()
                        // does not work:
                        .Map(m => m.nodeId, record => Guid.Parse(record.Values["nodeId"].As<string>()))
                        // does not work:
                        .Map(m => m.nodeId, "nodeId", EntityMappingSource.Property, o => Guid.Parse(o.As<string>()), true)
                    ;
                });
    }
}

[RichardIrons-neo4j]

Hi there.

The method that you have tried first here is not quite how the IRecordMapper<> interface is to be used. That is for mapping an entire record to an object, rather than mapping a single value to a particular type.

The second method you tried should work - however it doesn't, and you have uncovered a bug there. The default mapper tried and fails to map the guid value before your custom mapping gets a chance to take over. I should be able to get to this bug in the next few weeks.

In the meantime you could work around it by either defining a complete mapping from IRecord to Skill by implementing the IRecordMapper<Skill> interface, or perhaps use the delegate mapping in preview in the latest version of the driver, which would look something like

return result.Single().AsObject(
    (string nodeId, string skillName, int points) => 
        new Skill (Guid.Parse(nodeId), skillName, points)
);

I hope this helps tide you over until the bug is fixed.

[RichardIrons-neo4j]

@thedomeffm -I actually just thought of another workaround that could be less hassle to implement. You could use the IMappingProvider interface. Where you add the mapping stuff to the builder, you could change it to:

builder
    //.UseDefaultMapping() // this is where the bug is, so instead...
    //keep this line:
    .Map(m => m.nodeId, "nodeId", EntityMappingSource.Property, o => Guid.Parse(o.As<string>()), true)
    // and then do this for all the other properties of Skill:
    .Map(m => m.skillName, "skillName")
    .Map(m => m.category, "category");    // and so on...
    ;

Hope that's useful.

[thedomeffm]

Thanks for the answer, we will wait for your fix.