CosmosDBTrigger does not work properly with Graph model. GetPropertyValue<T> errors.

I have a Cosmos graph database (Gremlin.NET) and want to use Azure Functions with CosmosDBTrigger to utilize Change Feed but when I use Document.GetPropertyValue<T> I get a System.ArgumentException.

Repro steps

The following code errors on GetPropertyValue method call.

public static async Task Run([CosmosDBTrigger(
    databaseName: "MyDB",
    collectionName: "MyGraph",
    ConnectionStringSetting = "CosmosConnectionString",
    LeaseCollectionName = "Leases"
    )]IReadOnlyList<Document> documents, ILogger log)
{
    if (documents != null && documents.Count > 0)
    {
        foreach (var document in documents)
        {
            var lastName = document.GetPropertyValue<string>("lastName");
        }
    }
}

Expected behavior

Variable ‘lastName’ should be populated with string.

Actual behavior

System.ArgumentException thrown with message “Can not convert Array to String.”

Stacktrace…

at Newtonsoft.Json.JsonTextReader.ReadStringValue(ReadType readType)
   at Newtonsoft.Json.JsonTextReader.ReadAsString()
      at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)
         at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.PopulateObject(Object newObject, JsonReader reader, JsonObjectContract contract, JsonProperty member, String id)
            at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObject(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
               at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
                  at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
                     at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
                        at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)

Known workarounds

The only way I can get around this is to parse the object myself e.g. use JArray but that makes life hard and defeats the point of using Azure Functions Cosmos trigger.

I have also tried doing a ToString on the document object and deserialise like so…

var user = JsonConvert.DeserializeObject<User>(document.ToString());

The document.ToString() provides the following json…

"{
  "label": "user",
  "firstName": [
    {
      "_value": "Johnny",
      "id": "d0d8dbbc-8263-4f61-9bc1-60f8bcbd2c11"
    }
  ],
  "lastName": [
    {
      "_value": "Roberts",
      "id": "4a9e7f89-21a4-4424-859c-393400335695"
    }
  ],
  "userName": [
    {
      "_value": "jroberts10",
      "id": "3ed45581-e29f-404a-a75d-3279f5c48425"
    }
  ],
  "userStatus": [
    {
      "_value": "\\"GraphOnly\\"",
      "id": "d7fe7a21-979b-4633-9b5e-92c8f8331d72"
    }
  ],
  "primaryEmailAddress": [
    {
      "_value": "[e-mail]",
      "id": "5e13a000-6cf0-4413-b4fe-ea51d1c0bd2a"
    }
  ],
  "dateOfBirth": [
    {
      "_value": "01/01/0001 00:00:00",
      "id": "dd7556e4-9b08-4519-bdf8-aa94e92896a6"
    }
  ],
  "isCoach": [
    {
      "_value": "False",
      "id": "c3140fbb-4047-4cfe-a985-b810b25a5e84"
    }
  ],
  "id": "7b0c7dcd-e3df-4ee3-b324-1b8c009788a7",
  "createdDate": [
    {
      "_value": "11/02/2019 00:09:58",
      "id": "7644d23b-20c3-42a9-a40d-baa5357ee9df"
    }
  ],
  "partitionKey": "user",
  "documentType": [
    {
      "_value": "user",
      "id": "54aed62e-f607-450e-ab39-054a3d567a22"
    }
  ],
  "_rid": "Ix4TAMcT9pCZAAAAAAAAAA==",
  "_self": "dbs/Ix4TAA==/colls/Ix4TAMcT9pA=/docs/Ix4TAMcT9pCZAAAAAAAAAA==/",
  "_etag": "\\"00003b39-0000-0000-0000-5c60bd570000\\"",
  "_attachments": "attachments/",
  "_ts": 1549843799,
  "_lsn": 224,
  "_metadata": {}
}"

Related information

Package reference…

<PackageReference Include="Gremlin.Net" Version="3.4.*" />
<PackageReference Include="Microsoft.Azure.WebJobs.Extensions.CosmosDB" Version="3.0.*" />
<PackageReference Include="Microsoft.Azure.WebJobs.Extensions.EventGrid" Version="2.0.*" />
<PackageReference Include="Microsoft.NET.Sdk.Functions" Version="1.0.*" />
<PackageReference Include="Newtonsoft.Json" Version="11.0.2" />

[msivers]

@ealsur Just created this extension method to make it nice and easy in the meantime!

public static T GetProperty<T>(this Document document, string propName)
{
    return document.GetPropertyValue<dynamic[]>(propName).First()._value;
}

[ealsur]

@msivers I totally agree and it should be more easy for non-Core API accounts to consume the Trigger. We’ll work on adding more type binding support for those scenarios.