MLT JsonSerializationException : Could not create an instance of type Nest.IMultiGetOperation. Type is an interface or abstract class and cannot be instantiated. Path 'query.mlt.docs[0]._index'

[abibell]

Scenario

Elastic's More like this is good to find related documents for a documentId. As a business user I cannot know which fields make results relevant. It could be fields A, B & C today. I may realise that X & Y should be added when I am in production due to customer issues. What I want is that the way to configure the fields. In developer terms, a developer should be able to change the MLT query in a configuration and the system should use the query as template. In order to do this I should be able to serialise/deserialise MLT queries.

Working sense query

POST _search
{
   "query": {
      "mlt": {
         "fields": [
            "name", "content"
         ],
         "docs": [
            {
               "_index": "my_index",
               "_type": "my_type",
               "_id": "SOME_ID"
            }
         ],
         "min_term_freq": 1,
         "max_query_terms": 12         
      }
   },
   "size": 2
}

{
   "query": {
      "more_like_this": {
         "fields": [
            "title",
            "description",
            "abstract",
            "subject",
            "source"
         ],
         "min_term_freq": 1,
         "max_query_terms": 12,

      }
   },
   "size": 2
}

Test code that fails in MoreLikeThisQueryJson.cs

[Test]
public void TestMoreLikeThisQuery()
{
    Func<MoreLikeThisQueryDocumentsDescriptor<dynamic>, MoreLikeThisQueryDocumentsDescriptor<dynamic>> documentsSelector = d => d.Get("SOME_ID", op => op.Index("my_index").Type("my_type"));
    Action<MoreLikeThisQueryDescriptor<ElasticsearchProject>> selector = fz => fz
        .Name("named_query")
        .OnFields(f => f.Name, f => f.Content)
        .LikeText("elasticsearcc")
        .Documents(documentsSelector)//This causes the error
        .MinTermFrequency(1)
        .MaxQueryTerms(12);
    var s = new SearchDescriptor<ElasticsearchProject>()
        .From(0)
        .Size(10)
        .Query(q => q.MoreLikeThis(selector));

    var json = TestElasticClient.Serialize(s);
    const string expected = @"{ from: 0, size: 10, query : 
    { mlt: { 
        _name: ""named_query"",
        fields : [""name"", ""content"" ],
        like_text : ""elasticsearcc"" ,
        ""docs"": [
                    {
                       ""_index"": ""my_index"",
                       ""_type"": ""my_type"",
                       ""_id"": ""SOME_ID""
                    }
                 ],
         ""min_term_freq"": 1,
         ""max_query_terms"": 12
    }}}";
    Verify<SearchDescriptor<dynamic>>(json, expected);
}

private static void Verify<T>(string json, string expected) where T: class
{
    Assert.True(json.JsonEquals(expected), json);
    var returnedJson = TestElasticClient.Serialize(TestElasticClient.Deserialize<T>(json));
    Assert.True(json.JsonEquals(returnedJson), json);
}

Further Investigation lead to Conclusions

If we replace
Assert.True(json.JsonEquals(expected), json);
with
Verify<MoreLikeThisQueryDescriptor<ElasticsearchProject>>(json, expected);
in MoreLikeThisQueryJson.cs we get more test case failures.