Unable to map Hashset<object> while creating elastic index using Nest c#

Question

I am facing issues while creating index for my User model. My C# model is-

public sealed class User
{
     public long Id { get; set; }
     public string FullName { get; set; }
     public HashSet<Reference> References { get; set; }
}

public sealed class Reference
{
    public string Type { get; set; }
    public string Id { get; set; }
}

I am using NEST nuget for creating index.

var newIndexResponse = await _elasticClient.CreateIndexAsync(aliasData.NewIndex, i => i
                .Mappings(mappingsDescriptor => mappingsDescriptor
                    .Map<User>(m => m
                        .Properties(ps => ps
                            .Text(p => p
                                .Name(u => u.References)
                                .Analyzer(ElasticConstants.TwoLetterAnalyzerName)
                                .SearchAnalyzer(ElasticConstants.NameSearchAnalyzerName))
                            .Object<HashSet<Reference>>(p => p // This throws error
                                .Name(up => up.References)
                                .Properties(up => up.Object<Reference>(sp => sp
                                    .Properties(so => so
                                        .Keyword(eri => eri
                                            .Name(ei => ei.Id)))
                                    .Properties(so => so
                                        .Keyword(ert => ert.Name(t => t.Type)))
                                ))
                            )
                        )
    )));

When I try to run this code I am getting error for mapping Hashset.

Could not get field name for ObjectTypeDescriptor2 mapping: ArgumentException at Nest.PropertiesDescriptor1.SetProperty(IProperty type) at Nest.ObjectPropertyDescriptorBase4.<>c.<Properties>b__21_0(TInterface a, Func2 v) at Nest.Fluent.Assign[TDescriptor,TInterface,TValue](TDescriptor self, TValue value, Action`2 assign)

User model is a AWS DynamoDb entity so I am using Hashset in place of List. My end goal is to have search user by Type and Id in Reference class. I need help to figure out mapping of HashSet<Reference>.

Answer 1

I would rather use nested type instead of object for this case. That should help you with searching later on. You can read more about it here. NEST has a section in docs about configuring mapping you can check it as well.

Here is a working example (tested with elasticsearch 6.2.4 and NEST 6.8.1)

var newIndexResponse = await elasticClient.CreateIndexAsync("documents", i => i
    .Mappings(mappingsDescriptor => mappingsDescriptor
        .Map<User>(m => m
            .Properties(ps => ps
                .Text(p => p
                    .Name(u => u.References))
                .Nested<Reference>(p => p
                    .Name(up => up.References)
                    .Properties(sp => sp.Keyword(k => k.Name(n => n.Id)))
                    .Properties(sp => sp.Keyword(k => k.Name(n => n.Type)))
                )
            )
        )));

which creates an index with the following mapping in the elasticsearch

{
  "documents": {
    "mappings": {
      "user": {
        "properties": {
          "references": {
            "type": "nested",
            "properties": {
              "id": {
                "type": "keyword"
              },
              "type": {
                "type": "keyword"
              }
            }
          }
        }
      }
    }
  }
}

Hope that helps.