使用.Net SDK Microsoft.Azure.Cosmos查询Cosmos DB以获取不同派生类型的列表

Question

我们有一个接口和一个具有多个派生类型的基类。

public interface IEvent
{
[JsonProperty("id")]
public string Id { get; set; }
string Type { get; }
}
public abstract class EventBase: IEvent
{
public string Id { get; set; }
public abstract string Type { get; }
}
public class UserCreated : EventBase
{
public override string Type { get; } = typeof(UserCreated).AssemblyQualifiedName;
}
public class UserUpdated : EventBase
{
public override string Type { get; } = typeof(UserUpdated).AssemblyQualifiedName;
}

我们使用.Net SDKMicrosoft.Azure.Cosmos的v3将这些不同派生类型的事件存储在Cosmos DB的同一容器中。然后，我们希望读取所有事件，并将它们反序列化为正确的类型。

public class CosmosDbTests
{
[Fact]
public async Task TestFetchingDerivedTypes()
{
var endpoint = "";
var authKey = "";
var databaseId ="";
var containerId="";
var client = new CosmosClient(endpoint, authKey);
var container = client.GetContainer(databaseId, containerId);
await container.CreateItemAsync(new UserCreated{ Id = Guid.NewGuid().ToString() });
await container.CreateItemAsync(new UserUpdated{ Id = Guid.NewGuid().ToString() });
var queryable = container.GetItemLinqQueryable<IEvent>();
var query = queryable.ToFeedIterator();
var list = new List<IEvent>();
while (query.HasMoreResults)
{
list.AddRange(await query.ReadNextAsync());
}
Assert.NotEmpty(list);
}
}

似乎没有任何选项可以告诉GetItemLinqQueryable如何处理类型。是否有其他方法或方法可以在一个查询中支持多个派生类型？

如果有帮助的话，可以将事件放在某种包装实体中，但不允许将它们存储为属性中的序列化字符串。

Answer 1

Stephen的评论清楚地为我指明了正确的方向，并在这个博客的帮助下https://thomaslevesque.com/2019/10/15/handling-type-hierarchies-in-cosmos-db-part-2/我最终得到了一个类似于以下示例的解决方案，即我们有一个自定义CosmosSerializer，它使用一个读取Type属性的自定义JsonConverter。

public interface IEvent
{
[JsonProperty("id")]
public string Id { get; set; }
[JsonProperty("$type")]
string Type { get; }
}
public abstract class EventBase: IEvent
{
public string Id { get; set; }
public string Type => GetType().AssemblyQualifiedName;
}
public class UserCreated : EventBase
{
}
public class UserUpdated : EventBase
{
}

EventJsonConverter读取Type属性。

public class EventJsonConverter : JsonConverter
{
// This converter handles only deserialization, not serialization.
public override bool CanRead => true;
public override bool CanWrite => false;
public override bool CanConvert(Type objectType)
{
// Only if the target type is the abstract base class
return objectType == typeof(IEvent);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// First, just read the JSON as a JObject
var obj = JObject.Load(reader);
// Then look at the $type property:
var typeName = obj["$type"]?.Value<string>();
return typeName == null ? null : obj.ToObject(Type.GetType(typeName), serializer);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotSupportedException("This converter handles only deserialization, not serialization.");
}
}

NewtonsoftJsonCosmosSerializer采用用于序列化的JsonSerializerSettings。

public class NewtonsoftJsonCosmosSerializer : CosmosSerializer
{
private static readonly Encoding DefaultEncoding = new UTF8Encoding(false, true);
private readonly JsonSerializer _serializer;
public NewtonsoftJsonCosmosSerializer(JsonSerializerSettings settings)
{
_serializer = JsonSerializer.Create(settings);
}
public override T FromStream<T>(Stream stream)
{
if (typeof(Stream).IsAssignableFrom(typeof(T)))
{
return (T)(object)stream;
}
using var sr = new StreamReader(stream);
using var jsonTextReader = new JsonTextReader(sr);
return _serializer.Deserialize<T>(jsonTextReader);
}
public override Stream ToStream<T>(T input)
{
var streamPayload = new MemoryStream();
using var streamWriter = new StreamWriter(streamPayload, encoding: DefaultEncoding, bufferSize: 1024, leaveOpen: true);
using JsonWriter writer = new JsonTextWriter(streamWriter);
writer.Formatting = _serializer.Formatting;
_serializer.Serialize(writer, input);
writer.Flush();
streamWriter.Flush();
streamPayload.Position = 0;
return streamPayload;
}
}

CosmosClient现在使用我们自己的NewtonsoftJsonCosmosSerializer和EventJsonConverter创建。

public class CosmosDbTests
{
[Fact]
public async Task TestFetchingDerivedTypes()
{
var endpoint = "";
var authKey = "";
var databaseId ="";
var containerId="";
var client = new CosmosClient(endpoint, authKey, new CosmosClientOptions
{
Serializer = new NewtonsoftJsonCosmosSerializer(new JsonSerializerSettings
{
Converters = { new EventJsonConverter() }
})
});
var container = client.GetContainer(databaseId, containerId);
await container.CreateItemAsync(new UserCreated{ Id = Guid.NewGuid().ToString() });
await container.CreateItemAsync(new UserUpdated{ Id = Guid.NewGuid().ToString() });
var queryable = container.GetItemLinqQueryable<IEvent>();
var query = queryable.ToFeedIterator();
var list = new List<IEvent>();
while (query.HasMoreResults)
{
list.AddRange(await query.ReadNextAsync());
}
Assert.NotEmpty(list);
}
}